I had 3 separate sonograms to check my mirena (levonorgestrel) iud and it's perfectly in place. how likely is for me to get pregnant while having unprotected sex?

Very good question: I am impressed that you have the maturity to ask this question, and (moreover) that you were responsible enough to get a sonogram (three times!), I presume, to answer this question. I am not a pediatrician or a Gynecologist so I can only go by reported literature. Mirena (levonorgestrel) is 99% effective in preventing pregnancy for 5 yrs, so it works 99 out of 100 cases while in place for 5 yrs after implant.