Thanks to the great advice from this forum, I transplanted the 3 mystery irises to a sunnier spot in my garden and made sure to keep the rhizomes exposed. My reward: I see purple-looking buds on stems! This is the first time these irises (someone planted in my garden, probably to surprise me) will ever have bloomed. When they bloom, I will take pics to see if the forum folks can help me to ID the variety.