Trimarcisia
Trimarcisia, i. e., "feat of three horsemen", was an ancient Celtic military cavalry tactic or organisation; it is attested in Pausanias' Description of Greece, where he describes the use of trimarcisia by the Gauls during Gallic [invasion of the Balkans|their invasion of Greece] in the third century BCE.
According to Pausanias:
Pausanias' view was that the Gauls had adopted this method of fighting by copying the Persian Athanatoi elite force with the difference that while the Persians waited until after a battle to replace their casualties, the Gauls "kept reinforcing their full number during the height of the action".