- Unless you happen to have a problem of acidic hand sweat and burn through strings, you might as well put together a set of single strings from an online vendor or a good physical store that offers the option. The alternative, i.e. getting three strings from one set (provided such a set exists in the first place) and replacing the fourth one with a single, may be marginally less expensive, but entails finding a source of singles anyway (well, it is true that large-gauge single strings, usually used for low B, are easier to find compared to the rest of the range; but still).
- Assuming you'd spend an equal amount of time in all three tunings you've mentioned, I would choose "optimal" gauges for D standard, since each string would live in the relevant pitch for that tuning 66% of the time. Now,
-- what "optimal" means in your case depends on personal taste. Just as a rough guide,
--- 50-65-85-120 may be a good starting point if a firm tension is a priority
--- 45-60-80-115 (or 110) if you actually seek the typical tone of "normal"-gauge, downtuned strings.
- Regarding setup, have your guitar tech person do it with the bass tuned at midpoint (drop Db/C#), so that the result works as a passable compromise for all three tunings.
- Adjust to taste and priority, if your expect the amount of time *not* to be equal (in which case, playability/tone of one of the tunings might be sacrificed in favour of those of the other two).
To be honest, in your shoes I would just tune to drop C and use a capo on the 2nd fret when n