Well, I may not be able to offer any concrete suggestions, but I will offer some encouragement. I'm currently playing in a campaign where the party is composed of a wizard (played by myself), a druid and a sorceress. So far, it's been a blast, and I don't think our DM has really pulled any punches. We did rely a fair bit on the druid's animal companion which, until about 4th level or so, had more hps and a better AC then anybody else in the party, but it (being a snake) obviously wasn't as useful as having a full fledged fighter.
Given the makeup of the group, I expect that they'll probably annihilate the first encounter you throw at them, but struggle through the next, unless they have time to rest. In our group, we're pretty careful about making sure we can retreat, and have made extensive use of spells like rope trick to get out of tough situations and give us some breathing room. We do rely on NPC's from time to time, but have done some pretty brutal dungeon crawls without a fighter.
Summoning creatures works quite well too, but probably won't be effective until around 5th level or so, when the creatures actually stay around long enough to let you keep summoning more and more and overwhelm your opponents. In the last game we played, I used invisibility to get off 5 summon monsters (2 3rd's, 1 2nd, 2 1st's) and completely turned the tide of a battle.
Anyhow, I hope this helps, and I'll finish off with a shameless plug for my story hour, which may give you some idea of how a group like that can work.