Graphene lists all the pixel models they support and for how long. https://grapheneos.org/faq#device-lifetime

Finally had to ditch my 3a this month because Google is psychotic and it's too old for graphene or lineage. Sucks bc the phone still works great. I've temporarily got an 8a running graphene, and am loving it, but will be switching to a 9a shortly for non pertinent reasons. It is such a weird feeling of relief to be on a secure OS.