If a Nokia 6280 on 3 network is unlocked to use the O2 SIM while abroad can it be re-locked if you don't put the 3 SIM back?

When you have a phone unlocked it simply means that it will accept any sim card (any compatible sim card). So once its unlocked you dont have to worry about re-locking it. You can put your 3 sim card back in if you want, or an orange one or voda or keep the o2 one in. Its your choice.