You cannot introduce solids at 3 months. You need to check on things step by step.
1st see if your milk supply is enough is milk actually coming or your baby is just sucking and sucking and getting tired. (If milk is not sufficient then babies constantly cry and have prolonged sleep. I have personally researched on this as had same problem with my baby)
Use a nipple shield (am attaching nipple shield image) in this you can actually see milk coming. My baby is also 3 months old now and I use the same remedy for the 1st two months. And found out milk wasn't sufficient for him. So from then onwards I had to bottle feed him along with breastfeed. I breastfeed him and when my milk is over I shift to bottle immediately. And this I did only after consulting my baby's pediatrics. So it's safe.
2nd if milk supply is enough and still baby is troubling consult your pediatric they will check on 1st and then prescribe you drops to soothe your baby may be it's colic or stomach ache.