We don't know why flower buds are developing more slowly on one of the shrubs. However, there seem to be plenty of what appear to be healthy buds so the plant may catch up and grow normally. 

Are all three shrubs the same variety?  If not, genetic factors could explain the difference.

We suppose winter damage could account for the tardy plant's slow development too, but we can't be sure.
Years ago the flower buds of most forsythia varieties weren't winter hardy in Minnesota.  Bloom might be abundant after mild winters or on branches protected by snow, but often all flower buds were killed by severe cold and there was no bloom.  Otherwise the shrubs produced healthy leaves and grew well.

More recently forsythia varieties with flower buds that are reliably hardy, at least in Minnesota Zone 4, were introduced and these comprise most of the forsythia grown in the Twin Cities area today.