3 gallons is reallllly cutting it close :(
If you want I have an acrylic 5 gallon and a filter... maybe we can work something out, the light is burnt out though.
Most of the tanks you find online will be acrylic because of shipping. Not all, but most. Acrylic is more expensive than glass, and shipping kills a person. I haven't found many for less than $50 after shipping, and they are small.
If there is a possible way for you to fit a standard 10 gallon...
I think you are talking about Cyano as well, unfortunately it is very efficient at taking over a tank in a short amount of time. Astrea Snails will gladly help clean up.
It generally seems to be caused by excess phosphates and nitrates. As Len mentioned, controlling the nutrients with a...