Filtration only convert ammonia to nitrAte and pick up some waste off the substrate (moving water). It does not get rid of any dissolved waste in the water, which can be deadly if exposed in heavy amounts. This is why overstocked is never encouraged, especially in those cichlid tanks.
Its a 3g tank; however, substrate and decor displaces water, so you end up with approximately 2.5g of water or less, depending on how much water you displace.
Even if a DP is kept in a 3g, the keeper will have to do more than one 50% water change per week and test frequently. This is not ideal for most people. I'm a huge advocate of water changes, but I find it difficult to do more than one 50% water change per week. Its a hassle to do more than once and test weekly. Keep it as easy as possible for long-term success. Fish keeping should be super easy.