Given that green is not used I would recommend that you look to the artifacts that tap for mana which are legal for the environment that you are playing in. Since you give no indication which format you are going to be playing in here are some general examples; Mana Vault, fire diamond. You also have things like etherium sculptor which could be called mana ramping for artifact decks as it lovers the cost of other spells. There are also lands which will tap for more than one mana like tolarian academy. If you can give me more detailed information about which format you are wanting to use and why you are using these three colors I might be able to give you some more pointed examples to look at.
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Given that green is not used I would recommend that you look to the artifacts that tap for mana which are legal for the environment that you are playing in. Since you give no indication which format you are going to be playing in here are some general examples; Mana Vault, fire diamond. You also have things like etherium sculptor which could be called mana ramping for artifact decks as it lovers the cost of other spells. There are also lands which will tap for more than one mana like tolarian academy. If you can give me more detailed information about which format you are wanting to use and why you are using these three colors I might be able to give you some more pointed examples to look at.
What Format?
For Standard: Manalith, Sphere of the Suns, Horizon Spellbomb. Or for consistent mana growth, just use any of the card draw engines in Blue or Black.
Outside Standard: Everflowing Chalice, Knight of the White Orchid, Kor Cartographer, Pawn of Umalog.