Each of the three meanings can be paraphrased...
I [was in the habit of being] [accustomed to] [employing] it.
I assume OP wonders about the first meaning, but in reality I think it's just a tautological overlap with the second. It makes for an ugly sentence, to say the least.
The association of used with acclimatised over time, through repeated exposure or use seems unremarkable to me. I imagine the usage could have been re-coined repeatedly before it became a familiar part of normal speech and writing.
The expression used to in the sense of was in the habit of, has been around a very long time, as @Philoto assiduously researched. But originally it was as likely to be the present tense form use to as past tense used to. I think any such present tense usage today is simply by mistake, not in an attempt to convey 'archaic' connotations.
For some reason I can't really explain, the past tense form shot to prominence in the early 1800s.