I had been on SSRI for 3months but it cause sleepiness, I thought it is going to boost mood? Doesthat mean I don't have serotonin deficiency but other?

If it boosts your. mood stay on it and consider taking it at night, since its causing sleepiness. Consult with your prescribing MD first, but this is the common way of dealing with this pattern of side-effects. If you're not getting a mood lift, then consider changing to another medication. Again, consult with your MD before making changes. Serotonin deficiency is a little deeper and broader topic...
Unclear. Your SSRI, then, is not working, and is also causing sleepiness. It is not clear if you are taking a dose that would be expected to work; but even if so, you would have more sleepiness with a higher dose. You need to discuss a change in medicine with your doctor. Theories about serotonin and norepinephrine, etc. are useful, but your doctor's judgment is better.