Are there any attacks with a plurality of computing power?
In a certain sense, all 51% attacks are really plurality attacks, but you have to consider all honest users as forming a single "coordinated group".  (Which is true when you think about it: they are all following the intended protocol as implemented by standard software.)

Suppose for example that 30% of hash power is controlled by honest miners, 32% by Cartel X, and 38% by Cartel Y.  The honest miners are mining on the main chain.  Cartel X wants to reverse a transaction that appeared in some previous block A, so they are mining on a side chain that diverges from the main chain at block A-1.  Cartel Y wants to reverse a different transaction that appeared in a previous block B, so are likewise mining on their own side chain from B-1.

Given enough time, Cartel Y's chain will eventually be longer than either the "honest" chain or the chain produced by Cartel X, so it will become the official record.  Cartel Y will be successful in reversing their transaction, and Cartel X will not.

The 51% figure is based on the assumption that all miners who are not part of the attacking cartel are honest.  But if some of them are part of a competing cartel, then a plurality indeed suffices, assuming you count all honest miners as a cartel of their own.

In principle something similar could happen in the event of a blockchain fork, where honest miners could split into two or more non-cooperating groups mining on distinct chains.  In that case an attacker controlling more hash power than either group could mount a 51% attack also.