Just out of curiosity, why?
You get a monk and ele just for entering EotN, and you get a mesmer shortly thereafter. You don't get a necro hero until you are well into the Asura branch of the EotN story, and it requires considerable travel through Norn territory to even reach Asura territory, so you will be short one hero for quite a while if you don't use Vekk.
If you already have a necro, it can only be either Olias or MoW from Nightfall, which means that you must also have, at minimum, Koss, Melonni, Tahlkora, and Dunkoro. So you would be entering EotN with at least three monks, warrior, dervish, elementalist, and mesmer to choose from when building your team.
Even if you don't have NF heroes, playing EotN will also give you warrior and ranger heroes that you can't avoid getting, a sin hero if you do the side quests, and rit and derv heroes if you do the Norn Fighting Tournament. So you will have a considerable number of hero choices anyway.
So, why are you gimping yourself by considering only those particular three hero professions? There are times when things will be substantially easier if you make changes in your team lineup.
That being true, I personally think you're gimping yourself using Warrior/ Dervish hero's and sadly, a Necro can do most of what a Ritualist can do much better.
A Monk/Mesmer and Necro will do fine for almost all of EoTN, and if you do use Ele's, remember that there are quite a foes in EoTN that are immune to burning/ resistant to fire, namely Destroyers.
If you have all of the other campaigns then I'd advise looking up the triple necro setup's (Sabway and Discordway) as I have found that they handle most area's with ease and require only small changes to work well in different area's
.
