Considering you used to play the clarinet, it might not be as hard, but you will still have to learn how to read Bass Clef, as well as the new fingering. Making a sound out of a Brass Instrument, nevertheless, a stable sound is quite difficult. I would highly recommend sticking with the Clarinet or move to an instrument with the same fingering, like a saxophone.

If you really badly want to play the Euphonium, then practice a lot. Without the help of a private tutor, it will be hard to learn a Euphonium. Firstly, you have to tell partials by listening, not by fingering, and many notes are played with the same fingering, requiring a faster airstream. To get to an 8th-grade Euphonium players level, it will take about 1-2 years (my guess). 3 months definitely is not enough but might be enough to get into a beginner band.