Is a 3 GB USB enough for installing Ubuntu?

Not exactly what you were originally asking, but I really like the "Live" USB drives @Wasif_Hasan mentioned above. You can try your 3GB drive first, it might fit. Otherwise use a larger drive and create a "Live" boot drive. You can boot to Ubuntu without actually repartitioning your Windows machine for dual boot. This way you can test and see if all your hardware is supported (such as laptop camera). Also you can get a taste of Ubuntu before you take the larger step. This tutorial should help:

https://ubuntu.com/tutorials/create-a-usb-stick-on-windows#1-overview

You will get a slower experience, but if you like what you see there is a link on the "Live" desktop to do a full side-by-side desktop install (or replace Windows completely on that machine - but see below).

What ever you do, I suggests backing up all your data on your Windows 7 machine first. Good practice even if you are doing a major Windows upgrade. If your hardware and budget allow, I would also recommend moving to Windows 10 ASAP (as in one of the comments above), or just move to a new Windows 10 machine and completely replace Windows 7 with Ubuntu. I've always found a second machine to be more useful than a dual boot (although that's how I started).

You can, but it's not necessarily so straightforward. Most live images use a compressed root filesystem, which is much smaller than an uncompressed root. The obvious solution is to install less packages, replace the desktop environment with a lighter one, etc., but this is not always a reasonable solution. That's why you'll most likely need to use a compressed root too.

Some numbers: A minimal installation of most linux distros with a Desktop Environment and very common packages (office, music, video, browser, etc.) weighs around 3-5 GB. Because CDs only have a capacity of 650-700 MB, and binaries and text files (both which are the bulk of those 3-5 GB) compress very well, all live images of the distributions that I know of use a compressed root.

The easiest way to have a compressed root filesystem is to use btrfs with compression enabled (but beware that btrfs is somewhat less stable, especially if the system hangs or experiences sudden power loss). See this thread for ideas about how to install on btrfs with compression: Trick installer to use btrfs root with compression

Another way is to do it the way live images work, but this is much more involved: In many distros, live images have the root mounted as an overlayfs, which combines two mountpoints, with the bottom layer a squashfs image (a filesystem designed to compress very well files to fit distros onto a CD), and an tmpfs upper layer (a volatile, in-RAM filesystem) which holds the changes to the system.

Ubuntu Install Quick Start

The current Ubuntu 20.04 install disk is 2.7GB.

Download Ubuntu 20.04 https://ubuntu.com/download/desktop

Start with a USB of at least 3GB formatted FAT32.

If using Windows, download Rufus. https://rufus.ie/

Double click the Rufus .exe file. (No need to install).

Select USB Device and Ubuntu ISO file for Boot Selection.

Confirm Persistent partition size is 0 MB.

Select START

When Rufus is done you can use USB to install Ubuntu to BIOS or UEFI machine.

Yes and potentially No.

A netboot or mini.iso will fit on a 384MB thumb-drive/install media (possibly smaller depending on release should you have a thumb-drive that small still lying around).

The last Ubuntu desktop ISO I downloaded (groovy yesterday) was 2.5GB and I used a 4GB thumb-drive to write that onto.

But the size varies on release, and that was a groovy desktop ISO will be larger than older ISOs that contain less or no closed-source drivers, plus released (non-development) ISOs include less diagnostic/development libraries useful in crash reporting which exist only on development ISOs.

However my most common thumb-drives I use are still 2GB as many ISOs will fit on them.

The thumb-drive size required will depend on

• the release you want (closed source blobs aren't included all releases)
• the version you want server? desktop? or other?
• the flavor you want? (some don't include closed source blobs)
• plus the ISO you choose (options exist)

We have no specifics, so cannot provide a specific answer.

Yes 3 GB USB drive will fit for Live Ubuntu ISO. I have installed Ubuntu 18.04 (1.8 GB disk image) in multi boot with windows with 3.66 GB USB drive. If you use a mini ISO then the you even don't need a 3 GB ISO, and old USB drive about 386MB is enough. Though if you want to create persistent USB stick, then to create the persistent partition file, you need a USB drive greater than 4 GB.