Yes, the eating of algae is very common among mollys.
One thing to remember is that you aren't watching that tank 24/7. If you were to put a video camera on that tank and let it run 24/7 I think you would be surprised at the differences in your fish's behaviors.
There are a number of other possibilities here, but mostly it involves her being the only female with 2 males. That is never a good situation.
It is also possible your female is preparing to give birth. They tend to isolate themselves soon before it happens to help ensure the survival of some of their young.
As for knowing if she's pregnant or not for sure, there is no way to know except to wait it out. A female molly can have 30+ fry every 30 days. Because they have the ability to retain sperm in the body, if they mate with 3 or 4 different males, they can produce 3 - 4 spawns of 30+ fry each without having to mate again. Any female coming from a store tank holding both male and female has to be presumed to be pregnant. It is because of these breeding habits that it takes me 6 - 7 months to condition my female mollys for breeding, to be sure that the fry I am getting are from the male and female I have paired.
How big is your tank? Tank size and shape can also sometimes play a part in how males/females interact. The amount of decoration is important, but where its placed is also important. I will post a photo here for you of a properly decorated livebearing planted tank as an example. Take a look at the pic, then go back and take a look at your tank... considering the contents are different, are the same areas covered and just as heavily? What is a lot to one person may not be much of anything to another. I have found that out over the years first hand. When I refer to a large tank, that means anything over 120 gallons. Some people call a 30 gallon tank large.
The addition of more females should help, and clear photos of her and the tank will also help for us to see the situation first hand. Some things are more obvious to certain people based on past experience, and sometimes a photo can say everything.