I have GM SIL3 relay which says it has contact rating 3A, 125Vdc, 120Watts. I am having SOV on the output side powered by 24V dc.

1. Can I use this relay?

2. If I give 24V dc, will the current rating for the contact increase?

 

The relay contacts should be able to handle 24V dc. Whether they can handle more than 3A is a bigger question. You really should check with the relay manufacturer. They would need to know solenoid pick up current and steady state current (and know that it is an inductive load).

You should also determine the contact resistance: contact resistance times pickup current = voltage drop. 24 V - contact voltage drop - wiring voltage drop = voltage at the solenoid coil. You need to check on minimum pick up voltage for the solenoid.