3rd month at8.7kg is tip top high percentile! Well done mama.
It is still best to wait till baby 6 month then start him on semi solid. His tiny tummy may not ready for foods other than milk during first 6 month.
Talk to your pediatrician if your baby constantly seems not satisfying after nurse or after his milk feed.
Although 6 month is the general rule, there are exception that some baby may ready for semi solid earlier than that.
But 3rd or 4th mnth still very very early to start. If can, try to wait. They will have their entire life eating solid so no hurry. Mama milks is the best for the first year. Solid is just supplement for eating fun.