Potion effects in certain radius?

this is kinda ez question as you can use same command that we mostly use in the portals wich is /tp @p[r=3] and that will tp all the players withen 3 blocks of the commandblock so you cn basicily make it effect instead of tp and it will look like

/effect @p[r=15] minecraft:{any effect you want without the prakets}

Ex "/effect @p[r=15] minecraft:speed 3 60" that command will give all players withen 15 blocks away from te commandblock the effect speed level 3

and untill they leave the area the countdown from 60 will start so they will get the command allways as long as the player is inside the area then when he left it the countdown will start wich is the effect time

"note" set the commandblock to repeating so it willl be giving all the players withen the range the speed effect for 60sec and those 60sec will be regenerating

if you want it to make them lose the effect directly when they leave this area just set the time for 1

you spelt mining_fatigue like mining_fatiue