I've played it a few times. my best strategy is to build tack shooters 2 at the start then upgrade them to maximum one by one. then after getting 4 or so get a cannon (for lead bloons) after 8 or so get super monkey upgrade it to the max then get another. then get a monkey beacon so you can do the move which destroys all bloons on the map.
I FINALLY beat map 2 on medium. I was getting caught up in the idea that I HAD to have super monkeys. Instead, I focused on spamming with blade shooters. I must have had 20 blade shooters - they're cheap (compared to super monkeys) and very effective. The screen slowed down quite a bit during the last levels.
Took me a couple of tries to figure out that I had to get and upgrade a boomeranger pretty early, otherwise the level of all metal bloons will completely wipe me out.
Comments
http://www.ninjakiwi.com/templates/gameajax.php?name=Bloons%20Tower%20Defense%203&game=http://www.ninjakiwi.com/images/stories/Games/bloonstd3.swf
:P
My top is level 28 or 29. I had a Supermonkey and a Cannon.
Easy was too easy.
Finished 50 rounds with all lives, 3 super monkeys with the top upgrades.
I shall try medium next. I wish their scoring system was better like desktop defense - ie more points for sending early and an online leader board.
addicting never the less