Ah ha! If you are not tracking XP then you have the complete freedom to keep adding extra creatures as reinforcements when the need arises.
If the group are finding an encounter too easy (in your opinion) then bring in another wave flanking them or following up behind the first wave. If thats still not enough throw in some more.
This simple approach means that you can keep encounters dynamic and interesting while keeping everything under control. For important encounters I would look for interesting creatures to add if the need arises, or better yet design them yourself. I think a random (sort of) group of creatures stumbling into the battle site would make for an excellent surprise (for both the PCs and whoever it was they were fighting!).
You say your group is level 3, imagine everyones surprise if in round 3 of a fight 2 Death Jump Spiders leap from hiding and attack the back row.
Now this idea is probably a very good one. It can't be done all the time, but maybe half the time would work out...

Usually it's a bad idea to string the monsters along, it means the characters are only fighting parts of the encounter at a time. In other words, making it easier.
On the other hand, since I don't really care about the xp budget for any other reasons than trying to have an inkling of how difficult the encounter will be, I can start the encounter with 5 monsters and add 2 monsters in round 2 and 3. This means that instead of starting at 7 monsters and going down as time goes by, I will probably start at 5 - and stay at 5 for the three first rounds.
The added bonus of this is that the "new" monsters won't have any status effects on them allowing them to wreck havoc where I want them to.
Additionally since I am only controlling 5 monsters at a time, the rounds will be pretty quick.