What is the footprint of your tank?
Mollies get fairly decent sized. If you already have mixed genders, I probably wouldn't get many more for your tank and just wait for some mollie fry to come along. If your tank is decently long rather than just tall, then maybe get a total of like 5 (2 more in addition to the 3 you have) but I don't personally suggest getting more than that. In more cramped conditions, mollies tend to fail to thrive and die off in a matter of months IME.
Google Chinese algae eater and common pleco to see if you have one of those. They are both fish that are commonly sold under generic names like "algae eater" and "sucker fish."