I would get them each their own Legos to play with. Nothing would be more maddening to the 7-year old than to have the 3-year old take apart his creation! How would you feel if you created something and another adult took it apart?
You could do buckets. 7-year old gets his bucket, 3-year old gets his bucket and there could be a "community bucket" if you want.
If someone makes something then the other can't touch it without permission.
I am 100% for sharing, but not forced sharing. I would be very upset if someone forced me to share my things and kids are no different. I think teaching ownership is just as important. If something belongs to someone, it is their decision whether they want to share it. And the person who doesn't want to share has the right to say "no" but also know that a person who is stingy doesn't have many friends.
My brother was 4 years younger than me and I was upset when he used to destroy my creations. However, when we got older I then moved to the role of teaching him how to make his own creations. So give your sons time to grow up and it will improve as well.
Good luck!