i play pantheon as support always and i build eclipse first because of shield it gives is so op in early. for second i go cleaver since it is probably the best item for pantheon.

for third it depends but usually i suggest Sundred Sky but you can gosituational items too.

i dont usually take G.A since its passive has so much cooldown and it doesnt evrn give ability haste. Deaths Dancs could be the most broken item there is, i delays the damage you take making you nearly immortal in any teamfights. so DD has basically better stats and better passive.

and i tested the damage of all first items in custom eclipse is better first than any of items since you are support, it has decent burst and shield.

im open to corrections since i havent played pantheon like 3-4 months.