Two tenants now and one to be added later

Tenancy Deposits

Was just after someone's opinion on this. I have a 3 bed flat and am about to take on 2 tenants. They are happy renting for a month but have then said they want to find a 3rd person for the property. If I add the person a month in, what problems could you perceive there to be?

I plan on taking a full 6 week deposit from the 2 initial people (and they can sort out the deposit between them at a later stage). When the 3rd person joins I was going to add them on to the agreement, and then let them decide between themselves who pays what portion. As long as I get the rent I am happy. Of course I will do credit checks and all on 3rd person, but am I missing something?

Many thanks,

donncham