Partial Advancement from Trust

Where Trustees of a Will Trust want to advance to 3 of the 10 contingent beneficiaries (have to reach age of 25) and the Trust is considerably below the nil rate band, the whole of their presumptive share. Can the advancements be made by trustee resolution or is a deed necessary as the 3 beneficiaries interest will end.

Does a IHT100 need to be filed if the value of the transfer is less than 80% of the nil rate band and all the other excepted conditions are met.