How to check if a part's position is within a certain kind of box?

I don't know how to explain it,

let's say I have a shop and I want the shop GUI to popup when the player is standing in the 5 studs in front of the shop.

I heard about Region3 but the wiki couldn't help me, I don't understand it..

Would be helpful!

EDIT:

Something that would be cool:

local function checkifInRegion3(part,region3)
    local isInReg3=false
    --whatever code
    return isInRed3
end