What are the RX/TX pins the GSM library uses in the Mega2560

This is a silly question but I have problems with a GSM shield. It's RX and TX are clearly marked but software defines those in the Mega. I will cross TX and RX of course. That is, RX goes to the TX and vice versa. Arduino's documentation is meant to be easy but it confuses me when there is no mention of TX and RX.

So, when I use GSM library, what is RX on the Mega2560 and what is TX.

These are the pins used:

Transmission pin: 3
Receive pin: 10
Receive interrupt: 4