Did your ancestors have three children christened at once? Don't immediately assume that it was a multiple birth. Even in denominations that practiced infant baptism, for one reason or another, children of different ages might be baptized on the same date. Look for other records to confirm that the children were in fact baptized on the same date. They may have been twins or triplets, but confirm with other records when possible.
My grandmother and two of her siblings were all baptized in 1915 in a church which practiced infant baptism. They were all single births and why they were baptized on the same date I am not certain. It could easily have been a new minister, a realization that "it needed to be done" or some other reason.