I would leave the other two. Once they flower in June they will need to be cut back (after flowering). The bush will then produce new growth where next years flowers will grow. Did the frost kill the branches or just foliage. I would not cut back frost damaged bushes until after the danger of more frost is past because what is there offers a protective canopy to trap warmer air necessary to protect the lower parts of the plant. Deutzia is a fairly sturdy plant especially if it has been in for more than a season or two. In UK we have had such an odd winter weather wise that plants are very confused about what is going to hit them next. It has not been a typical winter and spring perhaps you are having the same trouble.