Should i separate my birds?
Yes, separating them would be best. The pair can seriously injure the lone guy and its best to prevent that. You should put the the pairs cage right next to the other ones cage so they can still see one another. If the lone one gets sad I would suggest looking into getting a same sex friend for him/her.
Its not advised to separate a bonded or mated pair because the birds can get stressed but if they are in separate cages but can see one another it shouldn't be a problem.
How big is your cage?
A cage for a pair of lovies should be 30' x 30' x 18' or similar. More budgies means a bigger cage. Also remove any nesting areas or paper as you do not want a 7 month old lovie laying eggs. That's too young.