Preparation is the key. Do you have somewhere to put them in (shed, stable) to load them into the trailer from? If not set the trailer up so you can create a pen behind it. Something sold and well secured. Pigs go through things and once they have done it once they will do it again.
Do they follow you for feed? Hopefully, the answer is yes to this one. In which case get them all to follow you to the loading area, feed in the trailer. Put all 3 in and then take two out. Practise this before loading day. If need be leave them until really hungry before you try it. Only give them a bit of food in the trailer so they will follow you back to the field.
Trying to take one from the group is unlikely to work. Aim to load the day before rather than giving yourself the stress in the morning when your under pressure.