Multiplication is a very old concept in math. So old that it dates back to ancient egypt, mesopotamia, etc. Each place probably came up with the concept on their own parallel to one another, or trade helped make the concept spread. It's very likely that the actual person who came up with the idea is unknown and won't be known since the concept is so ancient. It's like asking "who came up with the alphabet" which is an equally old concept.
Side note: multiplication is repeated addition. So if we added 3+3+3+3+3 to get 15, then we can say 3*5 = 12 as a shortcut. So its likely ancient peoples were adding the same number to itself over and over, leading them to develop a shortcut notation/method to get the answer quicker.
