The basics:

Below the Border (1942)
The Rough Riders, three friends who work on bringing justice to the West, are embarking on their new case involving a murderous criminal with eyes on a family’s fortune in jewels. Each of the Rough Riders poses as complete strangers to each other, in order to better investigate the case. When one of the Rangers goes under cover in the criminal’s gang, he finds himself in great danger and in need of his fellow Riders.

IMDb
Amazon – Below the Border
Amazon – Video On Demand – Below the Border
Amazon – Cowboy Classics 100 MoviePack

Here’s a clip of the opening three minutes or so:

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