3. If the positive integer $n$ makes $2^{1995}+2^{1994}+2^{n}$ a perfect square, find all such $n$.

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3. When $n=1994$, $2^{1995}+2^{1994}+2^{n}=2^{1996}$ is a perfect square.

When $n=1995$, $2^{1995}+2^{1994}+2^{n}=2^{1996}+2^{1994}=5 \cdot 2^{1994}$ is not a perfect square.
When $n>1995$, $S=2^{1994}\left(2^{n-1994}+3\right)$.
And $n>1995, \therefore n-1994 \geqslant 2$.
$$
\therefore \quad 2^{n-1994}+3 \equiv 3(\bmod 4) \text {. }
$$

Therefore, at this time $S$ is not a perfect square.
In summary, $n=1990$ or 1994.