Use the Basic Rules, it's fast and easy
Unless you want to spend time tracing lines at the table, just use the text in the parenthesis to infer three quarters cover
such as when the target is behind an arrow slit
Apply half cover if something is blocking the path (including one or more creatures), unless you think the cover is equivalent to being behind an arrow slit, then use three quarters cover.
The rules are different
It's important to note, the rules for cover in the Basic Rules or Player's Handbook are very different from the Dungeon Master's Guide rules for cover with miniatures.
With Basic Rules or PHB, an interposing creature grants one half cover.
A target with half cover has a +2 bonus to AC and Dexterity saving throws. A target has half cover if an obstacle blocks at least half of its body. The obstacle might be a low wall, a large piece of furniture, a narrow tree trunk, or a creature, whether that creature is an enemy or a friend.
A target with three-quarters cover has a +5 bonus to AC and Dexterity saving throws. A target has three-quarters cover if about three-quarters of it is covered by an obstacle. The obstacle might be a portcullis, an arrow slit, or a thick tree trunk.
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.
Because an interposing creature will nearly always block three or more lines. For an attacker, target and directly interposing creature of the same size, at least three lines are always blocked.
Only the most protective cover applies in the Basic Rules or Player's Handbook.
A further difference is you don't add multiple sources of cover in the Basic Rules or Player's Handbook, but this isn't a consideration in the Dungeon Master's Guide rules for miniatures and cover.
If a target is behind multiple sources of cover, only the most protective degree of cover applies; the degrees aren’t added together.
Multiple sources may combine for cover in the Dungeon Master's Guide rules for determining cover with miniatures?
It's difficult to tell, but the diagram shows three sources of cover that are apparently considered when drawing the lines. Nothing in the text says to treat them distinctly.
Playing on a grid, is this situation 1/2 or 3/4 cover?
Probably half cover
The introductory text for Using Miniatures does indicate
The Player’s Handbook offers simple rules for depicting combat using miniature figures on a grid. This section expands on that material.
Perhaps that means you still use the rules from the Player's Handbook. This would put limit on the degree of cover for multiple creatures to the maximum from any one creature, but would only matter when the creature was not directly interposing the attacker and target, where it would still provide three quarters cover using the DMG.