Colin,

For that third and last suggestion, you might need to offer some comments. What will the last be used for? Where do you have a gap? What do you plan to use the third rifle or gun for?

From memory you've got;
.22 Nornet/.410;
5,6x50R;
.22 Hipower;
,303;
.300;
.375,
.450?;
.500:
.600;
10-bore.

And others. I forget some of them!

Anything needed in the .243 to 7mm range?

Side by side, under and over, perhaps a combination rifle?