It would seem that your 3 year old is looking for attention and doesn’t really know how to ask for it. You might try to suggest that when they older one sees him coming he might ask if he wants to play with him. Then he can say, I just want to finish this so give me a minute, in the mean time think of something you would like to do.
Usually, kids just want to feel loved, wanted and needed. If you give them the attention they crave for they won’t do inappropriate things to gain that attention.