Are there custom guitar builders out there who produce 3 pickup guitars where all 3 can be used as one setting?

Perhaps some modification to the strat that includes a new setting or a wiring modification?

I really know nothing about this so I would appreciate some help.

you could use a 'neck on' switch
since you mentioned a strat i'll assume that's what you want to do it on
this is from memory but im pretty sure it's right
what you'd do is you would wire in an on-off toggle switch from the point on the 5 way where the pickup connects and the back of the volume pot (earth)
this would mean you position the 5 way as you would (for example bridge and middle) then turn the toggle switch on and all 3 pickups would be activated. it would also allow you to select the bridge pickup and flick the switch and use neck and bridge together

hope that helps.