That does not look like a rod shaped anal fin, properly called a gonopodium.
If this is the only molly with a fan shaped anal fin, this is the one which have birth. She doesn't look big enough to have more to come. There could well have been a lot more which were probably eaten - or they are hiding and you have yet to find them. Also, if she's young she won't have as many fry as a mature female. But she will probably have more fry in another 4 to 6 weeks.
But I do need to warn you that 1 female and 3 males is not a good ratio. Males will pester a female almost continually and with three males pestering one female she will become exhausted. The usual recommendation is 1 male, 3 females, though this does lead to a lot of fry.