Here is a picture for 4v4 corner play with tips.

For this to work in 3v3 the blue mid would move back to just in front of where the blue gk would normally be but still doing the same sort of thing as it tells you to do.
Also player 2 would move down to just diagonally up-right to the gk to block the cross. Then player 3 would move up to pressurise the blues gk/defender and get the clearances for a counter attack.
Whilst defending the two who are defending the cross/shot here's some help for you;
If you are the GK stay on the post until the cross comes in, if it is going nearer to the post than your defender then go for it, otherwise stay on the post depending on the situation, if the cross gets past the defender and the opposition looks like they are going to shoot try to block the angle by the post nearest to them; your defender should block the angle across goal otherwise its a >Blazingfag tbh :/
If you are the defender/mid blocking the cross then go for the cross or if you know you can't get it try to block the cross back across goal/shot from the opposition player.
Attacking from corners:
If you are the person in the corner either wall bounce or cross to your teammate down low/up high. If you get it to them and you see they are going to shoot try to get in place for another shot/cross if it deflects off the GK towards you or the surrounding area, this way you keep the pressure up and many GK's can only make one good save in a row and then are out of position leading to an easy goal (Blazing

If you are the one looking to get the cross make sure to keep moving to hopefully make the defender and GK move so your teammate has an easy cross and you have an easier clear shot. If the cross is not great and you have no option to shoot on goal then try to get the cross back to your teammate who can then hopefully shoot/cross again till you have an angle to score in.
Playing as the defender/GK you should try to look for clearances and feed them back in for an easy cross/shot to help keep the pressure up, if you think you will lose the wall fight with their FWD then back away to goal, THERE IS NO SHAME IN THIS! Just do not sit on the line (>Blazing).
As the two attackers/mids only pass back if their is no other choice or the opposition is bussing :/ Then get in position for the ball back to you to get that cross in!
Final tips for corner play:
1. Don't just spam running in.
2. If winning even if its by one goal just try to block the cross rather than pressure them as much.
3. If you are waiting for the clearance watch for the back pass.
4. If you are GK stay on the front post! (Unless otherwise needed)
5. Keep moving unless your the GK!