ffmpeg amix filter command execution never stops

Hello I want to achieve something like in this pic below using ffmpeg:

Initially I used below command:

ffmpeg -i input1.mp4 -i input2.mp4 -i input3.mp4 -filter_complex 
nullsrc=size=960x304 [base];
[0:v] crop=min(iw,320):304:(iw-min(iw,320))/2:0 [v0];
[1:v] crop=min(iw,320):304:(iw-min(iw,320))/2:0 [v1];
[2:v] crop=min(iw,320):304:(iw-min(iw,320))/2:0 [v2];
[base][v0] overlay=x=0:y=0[tmp0];
[tmp0][v1] overlay=x=320:y=0[tmp1];
[tmp1][v2] overlay=x=640:y=0[tmp2]
 -map [tmp2] -c:v libx264 output.mp4

It was working well but audio was missing in the output so I used below command to mix audios of input videos:

ffmpeg -i input1.mp4 -i input2.mp4 -i input3.mp4 -filter_complex 
nullsrc=size=960x304 [base];
[0:v] crop=min(iw,320):304:(iw-min(iw,320))/2:0 [v0];
[1:v] crop=min(iw,320):304:(iw-min(iw,320))/2:0 [v1];
[2:v] crop=min(iw,320):304:(iw-min(iw,320))/2:0 [v2];
[base][v0] overlay=x=0:y=0[tmp0];
[tmp0][v1] overlay=x=320:y=0[tmp1];
[tmp1][v2] overlay=x=640:y=0[tmp2];
[0:a][1:a][2:a]amix=inputs=3:duration=shortest 
-map [tmp2] -map  -c:v libx264 output.mp4

I don’t know what happened but the execution never stops. It keeps on going. I thought the input audio’s sample rate and channel types are mismatched, so I tried below:

ffmpeg -i input1.mp4 -i input2.mp4 -i input3.mp4 -filter_complex 
nullsrc=size=960x304 [base];
[0:v] crop=min(iw,320):304:(iw-min(iw,320))/2:0 [v0];
[1:v] crop=min(iw,320):304:(iw-min(iw,320))/2:0 [v1];
[2:v] crop=min(iw,320):304:(iw-min(iw,320))/2:0 [v2];
[base][v0] overlay=x=0:y=0[tmp0];
[tmp0][v1] overlay=x=320:y=0[tmp1];
[tmp1][v2] overlay=x=640:y=0[tmp2];
[0:a]aformat=sample_fmts=fltp:sample_rates=44100:channel_layouts=stereo,volume=1[audio0];
[1:a]aformat=sample_fmts=fltp:sample_rates=44100:channel_layouts=stereo,volume=1[audio1];
[2:a]aformat=sample_fmts=fltp:sample_rates=44100:channel_layouts=stereo,volume=1[audio2];
[audio0][audio1][audio2]amix=inputs=3:duration=shortest 
-map [tmp2] -map  -c:v libx264 output.mp4

but same result. The execution never stops. I think there is something with audio handling. Also currently if I remove audio portion and only output video, the output video stops when shortest of the video finishes. I want that the output video should have lenght of the longest video. The short clips should display their last frame until the longest video finishes. Any help regarding this?

PS: Logs from logcat:

ffmpeg version 3.3.4
     Copyright (c) 2000-2017 the FFmpeg developers
      built with gcc 4.9 (GCC) 20140827 (prerelease)

Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'input1.mp4':
      Metadata:
        major_brand     : 
    isom
        minor_version   : 
    512
        compatible_brands: 
    isomiso2avc1mp41
        title           : 
    1946246598843281
        encoder         : 
    Lavf56.40.101
      Duration: 
    00:00:53.91
    , start: 
    -0.022109
    , bitrate: 
    256 kb/s
        Stream #0:0
    (und)
    : Video: h264 (avc1 / 0x31637661), yuv420p, 400x304, 206 kb/s
    , 
    25 fps, 
    25 tbr, 
    90k tbn, 
    50 tbc
     (default)
        Metadata:
          handler_name    : 
    VideoHandler
        Stream #0:1
    (und)
    : Audio: aac (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 47 kb/s
     (default)
        Metadata:
          handler_name    : 
    SoundHandler
Input #1, mov,mp4,m4a,3gp,3g2,mj2, from 'input2.mp4':
      Metadata:
        major_brand     : 
    isom
        minor_version   : 
    512
        compatible_brands: 
    isomiso2avc1mp41
title           : 
    617689615538740
        encoder         : 
    Lavf56.40.101
      Duration: 
    00:00:15.37
    , start: 
    -0.022109
    , bitrate: 
    392 kb/s
        Stream #1:0
    (und)
    : Video: h264 (avc1 / 0x31637661), yuv420p, 400x400, 349 kb/s
    , 
    30 fps, 
    30 tbr, 
    90k tbn, 
    60 tbc
     (default)
        Metadata:
          handler_name    : 
    VideoHandler
        Stream #1:1
    (und)
    : Audio: aac (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 47 kb/s
     (default)
        Metadata:
          handler_name    : 
    SoundHandler
Input #2, mov,mp4,m4a,3gp,3g2,mj2, from 'input3.mp4':
      Metadata:
        major_brand     : 
    isom
        minor_version   : 
    512
        compatible_brands: 
    isomiso2avc1mp41
        encoder         : 
    Lavf57.25.100
      Duration: 
    00:00:05.97
    , start: 
    0.000000
    , bitrate: 
    770 kb/s
        Stream #2:0
    (und)
    : Audio: aac (mp4a / 0x6134706D), 22050 Hz, stereo, fltp, 128 kb/s
     (default)
        Metadata:
          handler_name    : 
    SoundHandler
        Stream #2:1
    (und)
    : Video: h264 (avc1 / 0x31637661), yuv420p, 368x480, 608 kb/s
    , 
27.29 fps, 
    120 tbr, 
    12k tbn, 
    60 tbc
     (default)
        Metadata:
          handler_name    : 
    VideoHandler
Stream mapping:
      Stream #0:0 (h264) -> crop
      Stream #0:1 (aac) -> aformat
      Stream #1:0 (h264) -> crop
      Stream #1:1 (aac) -> aformat
      Stream #2:0 (aac) -> aformat
      Stream #2:1 (h264) -> crop
      overlay
     -> Stream #0:0 (libx264)
      amix
     -> Stream #0:1 (aac)
    Press [q] to stop, [?] for help

ffmpeg -i input1.mp4 -i input2.mp4 -i input3.mp4 -filter_complex 
nullsrc=size=960x304,trim=0:1 [base];
[0:v] crop=min(iw,320):304:(iw-min(iw,320))/2:0 [v0];
[1:v] crop=min(iw,320):304:(iw-min(iw,320))/2:0 [v1];
[2:v] crop=min(iw,320):304:(iw-min(iw,320))/2:0 [v2];
[base][v0] overlay=x=0:y=0[tmp0];
[tmp0][v1] overlay=x=320:y=0[tmp1];
[tmp1][v2] overlay=x=640:y=0[tmp2];
[0:a][1:a][2:a]amix=inputs=3:duration=longest[audio] 
-map [tmp2] -map [audio] -c:v libx264 output.mp4