Read quickly, this is a joke, and a slightly backhanded love poem. The speaker tells someone they are a "back up tyre," the thing you keep in the boot in case something goes wrong. That's not the most flattering compliment, and the title, "Doubts III," hints that the speaker may not be fully sure about this relationship. The wordplay does the rest: "I will never tire of you" turns the tyre image into a promise of lasting devotion.
The repeated line, "It's best as a trilogy," frames everything. It could refer to the poem's own numbering, as if this is the third in a series, or to a relationship that works best with three parts, perhaps even three people. The poem doesn't say, and that ambiguity is part of its charm. The mood is playful, dry, and a little uneasy: affection delivered with a wink, while something less certain sits underneath.