Three inches is too deep. What kind of plants were you trying to grow and can you list your other tank(s) specs?
+1 to sifting. The quality of MGOCPM varies from bag to bag. Some bags will have more twigs and sticks while others will have more fines. The most recent bag I used had some pieces of gravel mixed in. The wet-dry cycles actually help to mineralize the organics so you get less rotting organics in the substrate. You can deal with substrate gas buildup in a smaller tank by using a bamboo skewer or chopstick. Decaying organics do release CO2 though, so it can be beneficial to have some in your dirt to a certain degree.Before you put your miracle grow into your tank make sure you sift out the large pieces of mulch, wood and stuff out of the dirt. You can leave small pieces just get the big pieces out. You can also take it and throw it into a long bucket or tupperware thing outside, wet it, cover it for a week and let it work it's own magic. A week later get it and put it into your tank to start your plants off right. The 2 most successful dirt tanks I have done, I put the dirt in, wet it lightly, let it dry, re-wet it, let it dry about 4 times and then actually started the tank with a cap, etc. Took awhile (2-3 weeks) but it was really worth it with the no worry about the dirt - and the WAY less mess when I moved the plants around because the dirt had already aged a bit and stuck together. Then weekly when you do your water change make sure you stick your finger or a stick into the substrate in lots of spots to ensure you don't have icky gas pockets building in the dirt under the substrate.
Another thing that helps reduces the production of hydrogen sulfide gas are healthy plant roots. Oxygen inhibits the anaerobic bacteria that produce hydrogen sulfide gas. If your plants are growing well, they will root into the substrate and oxygenate it.If you do it right, after 6 months or so you shouldn't have any nasty sulfur smell coming from the substrate, like rotten eggs, it should just smell fish tanky.