By adding up 3, 6 and 1, we get 10. Then the 3 in the given ratio corresponds to 3/10 of the whole (£250), or £75; the 6 corresponds to 6/10 of the whole, or £150; and the 1 corresponds to 1/10 of the whole, or £25.
You can show that £75, £150 and £25 satisfy the ratio 3:6:1, as proof.