Here are the words, from Johnson's "Scots Musical Museum" (1792):


THE CAMPBELLS ARE COMING

The Campbells are comin, Oho, Oho!
The Campbells are comin, Oho!
The Campbells are comin to bonie Lochleven!
The Campbells are comin, Oho, Oho!
Upon the Lomonds I lay, I lay,
Upon the Lomonds I lay, I lay,
I lookit down to bonie Lochleven,
And saw three bonie perches play.

Chorus:
The Campbells are comin, Oho, Oho!
The Campbells are comin, Oho!
The Campbells are comin to bonie Lochleven!
The Campbells are comin, Oho, Oho!

Great Argyle he goes before;
He makes his cannons and guns to roar;
Wi' sound o' trumpet, fife, and drum;
The Campbells are comin, Oho, Oho!
The Campbells they are a' in arms,
Their loyal faith and truth to show,
Wi' banners rattling in the wind;
The Campbells are comin', Oho, Oho!

Just what "perches" have to do with it seems to be anybody's guess. Presumably they symbolize something.