This problem has a completely elementary proof, using just high-school calculus and basic properties of divisibility.
Proposition: Let $n \geq 2$ be an arbitrary natural number, and let $p$ be a prime number such that $p \nmid n$. If the prime number $p$ has the form $x^2+ny^2$, for some natural numbers $a$ and $b$, then this representation is necessarily unique. (We can eliminate the condition $p \nmid n$.) (In the case of your problem you can replace it with $n=3$.)
In other words, for $n \geq 2$, a prime number either has no representation of the form $x^2+ny^2$, or if it does, this representation is unique, up to sign. (In the case of $n=1$, a prime number either has no representation of the form $x^2+y^2$, or if it does, this representation is unique, up to sign and permutations.)
Proof: Suppose on contrary that for positive integers $(a, b)\neq (c, d) \in \mathbb{N}^2$ we have:
$$a^2+nb^2=p=c^2+nd^2.$$
Then clearly we have
$$a^2\cong -nb^2 \mod p,$$
$$c^2\cong -nd^2 \mod p.$$
Multiplying respectively and vice versa together, we get that:
$$(ac)^2\cong (nbd)^2 \mod p,$$
$$-n(ad)^2\cong -n(bc)^2 \mod p.$$
Since $p \nmid n$ we get that:
$$p \mid (ac-nbd)(ac+nbd),$$
$$p \mid (ad+bc)(ad-bc).$$
Let $\alpha_+=ac-nbd$, $\alpha_-=ac+nbd$, $\beta_+=ad+bc$, $\beta_-=ad-bc$. What we get is translated in this way: At least one of $\alpha_{\pm}$ is divisible by $p$ (Some information in the congruences are extra, but I think it is better not to remove them to see the symetries.)
Now look at these two relations(Brahmagupta's identity):
$$p^2=p.p=(a^2+nb^2)(c^2+nd^2)=(ac-nbd)^2+n(ad+bc)^2=\alpha_+^2+n\beta_+^2,$$
$$p^2=p.p=(a^2+nb^2)(c^2+nd^2)=(ac+nbd)^2+n(ad-bc)^2=\alpha_-^2+n\beta_-^2.$$
We know that $p$ divides at least one of $\alpha_{\pm}$.
Claim: If $p\mid \alpha_+$, then $p \mid \beta_+$.
Proof: If $p\mid \alpha_+$, then we have $p\mid \alpha_+^2$. Also we know that $p\mid p^2$, so we can conclude that $p \mid (p^2-\alpha_+^2)$. By the first identity ($p^2=\alpha_+^2+n\beta_+^2,$), we can conlude that $p\mid n\beta_+^2$. Since $p\nmid n$, by Euclid's lemma we can conclude that $p \mid \beta_+^2$. Finaly notice that $p$ is a prime, so we can conclude that $p \mid \beta_+$.
Claim: If $p\mid \alpha_-$, then $p \mid \beta_-$.
Proof: If $p\mid \alpha_-$, then we have $p\mid \alpha_-^2$. Also we know that $p\mid p^2$, so we can conclude that $p \mid (p^2-\alpha_-^2)$. By the second identity ($p^2=\alpha_-^2+n\beta_-^2,$), we can conlude that $p\mid n\beta_-^2$. Since $p\nmid n$, by Euclid's lemma we can conclude that $p \mid \beta_-^2$. Finaly notice that $p$ is a prime, so we can conclude that $p \mid \beta_-$.
On the other hand we know that $p\mid \alpha_{\pm}$. If we consider these two claims, then we can conclud that one of the pairs $(\dfrac{\alpha_+}{p},\dfrac{\beta_+}{p})$ or $(\dfrac{\alpha_-}{p},\dfrac{\beta_-}{p})$ is a pair of integers.
Case (I): Suppose that $(\dfrac{\alpha_+}{p},\dfrac{\beta_+}{p}) \in \mathbb{Z^2}$, then form the first Brahmagupta's identity we get that:
$$1=\dfrac{p^2}{p^2}=(\dfrac{\alpha_+}{p})^2+n(\dfrac{\beta_+}{p})^2,$$
but notice that this equation has just only these solutions: $(\dfrac{\alpha_+}{p},\dfrac{\beta_+}{p})=(\pm 1, 0)$, which is equivalent to $(ac-nbd, ad+bc)=(\pm p, 0)$. Replace $c=-a\dfrac{d}{b}$ in the relation $ac-nbd= \pm p$, which gives you:
$$\pm p =ac-nbd=-a.a\dfrac{d}{b}-nbd\dfrac{b}{b}=-\dfrac{d}{b}(a^2+nb^2)=-\dfrac{d}{b}p.$$
Now look at these three ralation: $(a,b)\neq(c,d) \in \mathbb{N}^2$, $ad+bc=0$, and $\dfrac{d}{b}=\pm 1$. It is impossible. (Notice that the relation $ad+bc=0$ is equivalent to $\dfrac{c}{a}=-\dfrac{d}{b}$, so we can conclude that $a=\pm c$ and $b=\pm d$, ... to get a contradiction.)
(In this case whence we realize that $ad+bc=0$, we were able to get a contradiction, but for the sake of symmetry I write it in that form.)
Case (II): Suppose that $(\dfrac{\alpha_-}{p},\dfrac{\beta_-}{p}) \in \mathbb{Z^2}$, then form the second Brahmagupta's identity we get that:
$$1=\dfrac{p^2}{p^2}=(\dfrac{\alpha_-}{p})^2+n(\dfrac{\beta_-}{p})^2,$$
but notice that this equation has just only these solutions: $(\dfrac{\alpha_-}{p},\dfrac{\beta_-}{p})=(\pm 1, 0)$, which is equivalent to $(ac+nbd, ad-bc)=(\pm p, 0)$. Replace $c=a\dfrac{d}{b}$ in the relation $ac+nbd=\pm p$, which gives you:
$$\pm p =ac+nbd=a.a\dfrac{d}{b}+nbd\dfrac{b}{b}=\dfrac{d}{b}(a^2+nb^2)=\dfrac{d}{b}p.$$
Now look at these three ralation: $(a,b)\neq(c,d)$, $ad-bc=0$, and $\dfrac{d}{b}=\pm 1$. It is impossible. (Notice that the relation $ad-bc=0$ is equivalent to $\dfrac{c}{a}=\dfrac{d}{b}$, so we can conclude that $a=\pm c$ and $b=\pm d$, ... to get a contradiction.)
Remark: The condition $p\nmid n$ is not a serious condition. Because if $p\mid n$ and $p=a^2+nb^2$, then necessarily we have $a=0$, and by simple argument we can show that $b=1, n=p$, and the representation is unique clearly.
The proof for the fact about $n=1$, is similar.
