A three-pounder saluting gun was small enough for a swivel mounting

Would a three-pounder saluting gun be too small to serve as a swivel gun?

No; the size is plausible for a swivel gun, although the precise lineage of the observed piece remains uncertain.

A three-pounder would be large enough for use as a swivel gun. Small saluting cannon also have a historical relationship with swivel-gun forms. Without the exact specimen or its service history to hand, the identification remains informed and tentative. A gun's small ceremonial use does not determine the mounting type on which similar weapons could serve.