I would recomend Slowbro out of the 3. As you say, Milotic will get worn doen by Sandstorm + Flame orb. Rotom-Wash has the problem of having a statline of having to either be fully offensive or fully defensive and sacrifice the other, which will add to your concern about pasivity.
Slowbro though, takes the Sandstorm damage easy thanks to Regenerator, and thanks to Calm Mind, isn't passive. Try running a set like
Slowbro @ Leftovers
EVs: 252 HP / 252 Def / 4 SpD
- Calm Mind
- Slack Off