Could be any number of reasons and you don't give any information. What's your zone? Are two of the three in more shade - full sun or a minimum of a half day of sunlight is ideal. Did you prune them possibly cutting into the old wood that produces the next season's blooms? Flower buds for the following year are set shortly after the plant is through blooming. Are you over fertilizing or fertilizing at all? Lilacs don't need it. Is their location too wet; are you over watering?