A man is bound to protect girls and take care of them--they can jolly well take care of themselves really it seems to me--still, this is what Albert's uncle calls one of the 'rules of the game', so we are bound to defend them and fight for them to the death, if needful.
Denny knows a quotation which says-- 'What dire offence from harmless causes springs, What mighty contests rise from trefoil things.' He says this means that all great events come from three things--threefold, like the clover or trefoil, and the causes are always harmless.

Trefoil is short for threefold.
There were certainly three things that led up to the adventure which is now going to be told you.

The first was our Indian uncle coming down to the country to see us.

The second was Denny's tooth.

The third was only our wanting to go hunting; but if you count it in it makes the thing about the trefoil come right.