The GM would assign control of all of the various players sheets to the one player account (the account you use to log into the table) which would mean that using the one PC you could have access to, and control of, all of your characters. It would potentially cause an issue in that you would always be able to see what all three characters can see even if on different pages or in different parts of the map which could be solved by Aaron's second suggestion of having three separate Roll20 accounts logged in at once using different browsers, browser accounts or the like (ie. 3 tabs of Chrome all logged into a different profile or a chrome tab, an incognito chrome tab and a firefox tab all logged into accounts on Roll20). The simplest solution in this case is likely the best, have the one player account (cody m.) assigned to control both (or all three if your daughter plays too) characters. It would reduce the amount of messing about during play and as long as the GM was cool with the shared vision then there shouldn't be any game ruining issues. As Aaron suggests, talk to the GM, they'll know how they can set this up to suit your needs and help you out... failing that, keep the questions coming, helping people is what this community is best at =D