Never have I saw, nor even heard of that being done. You have 8 gaps to cover on defense(9 if you consider an E gap wide of the TE side) and 11 players on defense, all of which have run responsibilities. Asking one player to cover nearly half of those is a pretty impressive assignment. Typically gaps are as follows
A - two gaps between center and guards
B - Two gaps between guards and tackles
C - Two gaps outside of tackle
D - Two wide areas outside the tackle box
In order to have a three gap assignment a DE lined up over the weak side tackle would have to take responsibility for anything outside of the guards outside shoulder, or nearly half the field. In a 4-3 weakside typically what you will see is the nose tackle/1 technique taking the A gap, The end playing either the B or C gap with the weakside backer taking either the B or C, whichever the End doesn't take. Corner takes the wide D. The free safety pending the scheme will often not have a gap assigned to them, but are expected to make a read and play the whole field.
Strong side your middle backer takes the A, 3 technique tackle takes the B, and the C or D is taken by either the End or Strong backer, pending where you have your players aligned. Corner plays the wide run, and once again the safety makes a read and gets to the ball.
Of course pending lineup/ play call these responsibilities change, but in no way can I see covering three gaps with one player. Not only would a player be expected to cover a large area of the field, but he would most likely be double teamed as well.