id cut them right at the base and pull them out from the bottom . i wonder more importantly why they died . to replant id reconstitute the soil . better if you can get the roots out but that mite prove hard . then plant young ones wear you can within the footprint of the established hedge .

Agree with Noseypotter ... why did 3 of the 20 die? Were they all at one end, or in a block, mid-hedge? Were they also all planted at the same time? Have you checked the soil for grubs and another nasties eating the roots?