What is the minimum conduit size for 3 -#2 copper wires to a service panel?

Always needs to know the insulation, as different types have different outside dimensions.

Assuming THHN/THWN insulation, 1 inch is the minimum.

Electricians never use 1 in GRC for 3 #2 wires. We will always use a min of 1-1/4".

Just the way we are.

Might you need 4 wires. L1, L2, G and N?