Alolan Exeggutor is a better Exeggutor in its role as a Grass Attacker, as the added Dragon typing helps Alolan Exeggutor resist the water type attacks you're bound to receive while using its Bullet Seed Solar Beam moveset. Barring the community day exclusive Venusaurs, A* Exeggutor is a top grass contender. So yes, power that up.
Your only hesitation should be to consider whether now is the most relevant time.
Sure Milotic is a Water type and a great defender, but the walking wall and general added hassle of evolving a feebas may be a deterrent to seeing them frequently to matter for you to have a Grass attacker. In short, A* exeggutor was great to have during the Kyogre/Water Week, not so much now.
I can't speak for the other two. Would need to do more research.