i dont know much about these but i planted some one year, they had gone the next ~ i think the frost killed them, could it be that yours suffered last winter?
ive given up trying to grow them because i have too little sun in my garden, could it be that you need more sun for yours to flower??

Three in one pot may be getting quite crowded now, unless it is a really big pot.

I've found that Agapanthus need what passes for a really good baking here in the UK before they'll flower. And they like to be crowded in a pot, so I don't think that should be a problem yet. So I'd put it down to the succession of poor summers here. If we have a hotter summer next year your agapanthus may put on a better show the year after.

I don't think there is any particular rule that Agapanthus follow. Any I have planted it the garden, crowded or not just vanish or fail to bloom. They have bloomed in crowded pots and in uncrowded pots. Got frosted or not frosted next to one another. Bloomed one year and not the next. I see them in other places relatively neglected, in all their crowded glory, blooming year after year.....infuriating. Just think how pleased you are when they do as you want them to do.