Make sure the density of the bulks that you're using, and the size of the capsules you're using will even allow for 900mg per cap. At 900mg you may have to use 000 caps, especially if you're using a less dense powder.
It should go without saying that you should get a mortar and pestle or at least a plastic baggy so that you can try to evenly mix the 3 powders.
You should also make sure you've got a movie or something to watch because depending on how many you're making, it can be a little time consuming. Might as well enjoy it.