That's why there's that 'maybe' with Hostage Taker. If I don't run it, I might get away with replacing a few Forests and Swamps with the rare UB/UG duals and run a single Island for Evolving Wilds/Field of Ruin/District Guide. If I do run it, I need a few more U sources; thus might have to start including Gates. That said, it is quite likely that Hostage Taker is good enough to justify that; especially given that Green provides additional fixing.
3-colour manabases are tricky to figure out in Singleton. Green has the easiest time because they can run additional fixers like
District Guide,
Open the Gates and the likes. If you want to run 3-colour without Green your best best is something like:
12 Duals (Shocklands, Checklands, Gates, tapped Duals; roughly in order of importance)
Evolving WildsGateway PlazaRupture Spireplus the Basics you want. This gives you 11+#Basics sources of each colour - not great but about the best you can do. There's some cards that still can provide additional fixing, but overall this also means that you may not be able to cast cards with heavy mana requirements like
Mass Manipulation or
Frilled Mystic reliably in 3-colour singleton decks, so you should think twice whether you really need them or whether you can replace them well enough.