Firing a 3" Mortar.

If I want to fire a 3" mortar , need a spotter? The mortar cannot self spot?

The procedure is

1- roll to request fire 3+
2- priority check , 3+
3- Comms check 3+

Then I get to place a spotting round?

No.

If you are firing a specific weapon, in this case a 3" mortar, you should have paid points for it either from the Platoon support options, or from 'Artillery'. In which case, on or off table, single weapon or battery, it's Comms check - Spotting round.

The Priority check is part of the Off-Table Artillery Request from 'Additional Fire Support' rules, and does not have fixed values, it depends on the army list in question for the Priority check, as well as what priority level you paid for, and the level at which you are requesting fire from determines the Comms check.

You cannot self-spot for medium mortars.
Procedure is as usual:
1. contact battery or single mortar. Tha’s a comms check. Usually 3+
2. Place spotter round. Roll for accuracy and deviation.
3. Fire for effect

Ok thanks
So I can skip step 2 and 3 if the mortar is part of the platoon.