Superphosphate is used in the planting hole with a buffer zone of soil between where it's been dug in and the iris roots, to prevent burning. 3 years to begin blooming is not unusual.

About 6 weeks before your bloom time an all purpose fertilizer may be used. Like a tomato or rose fertilizer. Lower in nitrogen than some others. Then no more (unless you have rebloomers). If you have sandy soil you may need to apply 2 applications in spring before before bloom.

Lack of enuf sun will prohibit bloom as you and I have both discovered. For those in full sun that skipped blooming this year but are looking healthy ---- give them time to build a clump. Newer hybrids with their hugh flowers are not like the iris of our grandmothers'. Their babies are probably taking 2 years to become big enuf to bloom. So babies started last year will bloom next year.

Schreiners has done a great job on information at their site.

https://www.schreinersgardens.com/home/scg/how-to-grow-care-for-bearded-iris