I'd call 1756 grenadier a 45mm figure, though whoever made it may have have called it 54mm. It's definitely not a Britains figure, old or new. The closest thing I've ever seen is Authenticast, but I won't say it's an Authenticast figure, either. It doesn't look to me like Holger Erickson's style of sculpting.
As for the other two, if there are no marks under the base, no raised lettering with "Copyright", "Made in England", or similar marks we can see with Britain's toy soldiers, then I don't think they're Britains, either. Are they hollow or solid?
Prost!
Brad