Can re-ring pistons with out take heads off '' mitsubishi 1994 Diamante 6 cilinders SOHV 3.0

NO!
A ring job will involve honing the cylinder walls to make them ready to accept the new rings. It also should involve measuring the bores to make sure that they are round and not scored too badly to not allow a good seal. Then the rings are placed on the pistons and then assembled through the top of the block.

After removing the ring ridge, which can only be done from the top