Dormammu is a completely different league to the other two, he should solo
He really really isnt. Dormammu outside his domain is a skyfather being apparently. And eternity he beat in Strange Tales 146 has been religated to an M-Body of the actual Eternity who apparently though that incident was dismissiable. Dormammu only defeated him with Umar's powers. Dormammu by his own admission is equal to Odin and Zeus in power level, putting him soundly below Galactus.
Galactus has a good chance, but i know little about Nabu apart from him going a distance in the fight with Spectre, and if he is indeed in that level to hold his own, albiet for a short time against Spectre, he would be above Galactus.
With Nabu it really depends on form. Based on the image in OP it's energy form Nabu who can hold his own with Spectre. in energy Form he's basically above classic Fate (who is classic Strange level). Classic Fate himself lasted several pages in battle with a Spectre who was amped by the ring of life. Host or mortal form Nabu is much lower. Another showing for energy form Nabu was when in he was punished and converted to a lord of Chaos. H soloed Fate, Alan Scot, Phantom Strangernand Zatanna ( IIRC). I've always seen Dormammu as well above odin level but I've only seen two appearances outside his own realm and it sounds like you just mentioned one