MArcomage

Interesting concept. However, I wonder if it can be practically used. In most games that condition is never fulfilled, with the exception of complete stock destruction. In that case even if you draw rare it won't help you much, I think.

Imps cache might help...

One deck type might be able to wield it: 0-cost Desolator...
Even then it will be tricky indeed.

or Order of White Lotus migh do the trick

That's even more unlikely then achieving it by Merchant.

What if the card itself allowed you to change your resources after it is played? Like:

Mode1: Bricks: -1
Mode2: Gems: -1
Mode3: Recruits: -1

Old version:

If bricks = gems = recruits:
Next card is rare
Always:
Tower: +1

New version:

If bricks = gems = recruits:
Next card is 0-cost rare
Always:
Tower: +1
+1 random resource

The fact that the next card is zero-cost means it will be relevant in anti-brigand situations. The +1 random resource condition makes it easier to think of using the card when the condition isn't fulfilled, and also prevents multiple uses of the card in a row.

Perhaps make it Durable as well?

Why do you want a Durable Common that will only occasionally be useful? Even in a Zero-cost deck, it would prove annoying in short order.

Make it durable so you can achieve more easily the conditions to get a rare (since it has the +1 random resource)

If this were Durable, you'd end up with multiples of it on hand by standard draw after playing other cards, and you'd never draw the promised Rare when playing it because keywords occur after card effect.