Installing dimmer switch there is hole in wall but no box, their was a switch their attached to dry wall
This is a modular home. Should I add a cut in box then put switch ? And there is three wires no 2 just tie all whites together and all blacks and grounds right?

Not sure what you mean by "should I add a cut in box " By code your switch needs to be in a box, you can add a box if that's what you mean by cut in. White wires are not associated with a switch, the switch interrupts the run of a black wire. You have to figure out which black is hot and which one continues on to the switch, those are the 2 connected to the switch

Mobile homes often use switches and receptacle devices that are modular and do not need outlet boxes.

If you add new, and are unable to find the modular style devices, and use standard devices, then back boxes will be needed.