What is the highest kv/lowest turn motor that can safely be run on 3s
im talking 540 size and for example ill say in a 2wd short course truck

Also what is the highest rpm that can be safely achieved.
I have a 3.5t and on 2s it theoretically turns 69,634rpm
on 3s it would potentially put out around 104k
is that too much?

It depends on the motor... castle 2 pole motors are rated for ~90k

Castle. 1406 4 poles are rated for 100,000 rpm. You will never hit that though.