Security implications of using hash as a MAC
In general, the construction $H(K||M||N)$ where $H, Y, N$ are known to an attacker is arguably no more secure than $H(K||M)$, which we already know to be broken. For that latter construction, we can easily forge a MAC of the form $K||M||PADDING||Z$ for some message $Z$.

This same attack would apply to the construction proposed in the question, except the forgery would be of the form $K||M||N||PADDING||Z||N$. This attack is feasible on many currently used hash functions, however, is not feasible on the new SHA-3.