The cover rules in the PHB say:
There are three degrees of cover. If a target is behind multiple sources of cover, only the most protective degree of cover applies; the degrees aren't added together. For example, if a target is behind a creature that gives half cover and a tree trunk that gives three-quarters cover, the target has three-quarters cover.
However the DMG (250/251) appears to have an alternate method for resolving multiple sources of cover:
To determine whether a target has cover against an attack or other effect on a grid, choose a corner of the attacker’s space or the point of origin of an area of effect. Then trace imaginary lines from that corner to every corner of any one square the target occupies. If one or two of those lines are blocked by an obstacle (including another creature), the target has half cover. If three or four of those lines are blocked but the attack can still reach the target (such as when the target is behind an arrow slit), the target has three-quarters cover.
The only case I can think where these might collide is something like the following:
Individually T would not get cover from D1 or D3 and only get half cover from D2. The DMG seems to indicate T might get full cover since all 4 lines would be blocked. However if you use the guidance from the PHB (as shown above) T would only have half cover.
Seems to me it's just a judgement call which interpretation is correct. Anyone know of any rules that clarify this other than a judgement call?
EDIT: assume D1, D2, and D3 are separate obstacles or monsters.
Maybe this makes it more clear: