The card Smelt//Herd//Saw from Magic: the Gathering has the cost of R/5G/1B.
What is its mana value? Possibilities I see include:
The Comprehensive Rules do not have a ruling for multiple split cards.
The mana value of Smelt//Herd//Saw would be 9.
Smelt//Herd//Saw is a test card that is not legal or intended for serious play:
Playtest cards aren't legal for play in any tournament format other than Mystery Booster Limited formats. On the other hand, we expect they will spice up a wide variety of casual games. Cube may never be the same.
The comprehensive rules don't have explicit rules for split cards with more than two faces because there are no such tournament-legal cards. However, modifying the existing rules for split cards is straightforward:
For many playtest cards, you'll need to make a generous assumption that basic game rules will be updated to allow them to work.
709.4b The mana cost of a split card is the combined mana costs of its two halves. A split card’s colors and mana value are determined from its combined mana cost. An effect that refers specifically to the symbols in a split card’s mana cost sees the separate symbols rather than the whole mana cost.
The logical modification would be to combine not "its two halves", but "all of its faces" or similar, making you add the mana values of all its faces together for an MV of 9 outside the stack.
As stated, the mana value of the card is 9. However, when it's on the stack, it's a spell, and that spell has a mana value of 1/2/6, depending on the option chosen when casting.
This is to add to Hackworth's excellent reply, I know that you asked for the mana value of the card, and Hackworth answered that perfectly, but some players unfamiliar with split cards and mana values don't realise that spells and cards are different items for the game to recognise. This is particularly important given that a huge amount of cards that care about mana value want the spell's mana value; others want the card's!
(I would have added this to Hackworth's answer, but I don't have 50 reputation to do so, I'm afraid. )
