You might have to wait 30 to 45 days for your tank to completely cycle. Buy an AP liquid reagent test kit now if you don't have one, and measure the pH, ammonia, nitrite, and nitrate. Then every day or two measure the ammonia. If you have detectable ammonia, measure it daily to make sure it doesn't get high. After a week or two, or a few days of having ammonia, check the nitrites. If none, check them every day or two. Once you have them, check daily to make sure they don't get too high. When the nitrites go back to zero, check the nitrate, it should be higher than your initial check. With three Zebras in a 29
gal tank, it might be possible to cycle the tank without ever getting detectable ammonia or nitrite (ie: levels>0.25). This would be ideal and very kind to your fish. You will know the cycle is done because after a month or so you can check the nitrates every day or three, and when they increase, your cycle is done even if you never saw ammonia or nitrite. I have done this before with low bioload cycling. My link is at:
http://home.comcast.net/~tomstank/tomstank_files/page0017.htm
The hardest part is to resist adding more fish! If you feed sparingly, add no more fish (Maybe two more danios if no ammonia or nitrite after two weeks, maybe!), you can establish a biofilter without ever seeing ammonia or nitrite spikes. This low bioload approach is essentially a "cycle-less" fish cycle. Good luck!
edit: oh yeah! and when its done, add fish slowly, one or two per week to keep a cycle from popping up as you go to full stock.