Yes three would be great! Just a side note: when you shop for Otos- ask how long they've had them. If they've had them less than a week don't buy them. Check for round tummys, this means their eating good.
acclimate them to your water very carefully. Otos are caught from their wild habitat and treated very roughly thats why a lot die when we bring them home. By checking that they have survived at the fishstore at least a week and their tummys are round you are more likely to get healthy fish.
www.otocinclus.com is a great place to learn all about them. Good luck!!
Carol
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It probably is an algae. There are several different kinds. Each scavenger deals with their own specific favorite. So if you try another scavenger, I would think this would take care of it. I do not suggest EVER getting a chinease algae eater. They get big and aggressive toward other tank mates, and stop eating the algae after they get bigger. A dwarf Pleco would help, but might still be to big for a 10 gal. Otos are a really good way to go, get several. They are small work together, and are much happier this way. They clean off leaves, decorations, and the glass. Corys are more help cleaning left over food that falls to the bottom, not cleaning algae. 
