Merge 3 title deeds back into 1

As the title says, I am trying to buy a property as a residential mortgage but because there are 3 title deeds for the 3 inner flats within a terrace house, no body is willing to lend, therefore I need to merge/amalgamate the 3 title deeds into 1, the seller is happy to do this so long as its a simple process. How do we do this? And how much time roughly would it take? Can it also be done during transfer of purchase?

Raj - this thread explains how to apply for an amalgamation - Amalgamating titles · HM Land Registry
Timescales are very stretched so if there's an urgency you need to request expedition after you have made the application - Request an expedite - GOV.UK
Amalgamation can be requested as part of another transaction such as a purchase

So I should do it myself as the buyer or should the seller do it? Can a conveyencing solicitor do it?

Raj - very much a choice for the three of you to make. And any one of you can request it 

1. I want to do it as part of the transition/buying process so that the seller doesn't do anything, it should be me doing it as a condition on transfer, is this possible?

2. Is it £40 for the entire application or £40 per title merge i.e 3 x £40 ?