Triangle $ ABC$ has fixed vertices $ B$ and $ C$, so that $ BC \equal{} 2$ and $ A$ is variable. Denote by $ H$ and $ G$ the orthocenter and the centroid, respectively, of triangle $ ABC$. Let $ F\in(HG)$ so that $ \frac {HF}{FG} \equal{} 3$. Find the locus of the point $ A$ so that $ F\in BC$.
1. **Identify the key points and their properties:**
   - Let \( B \) and \( C \) be fixed points such that \( BC = 2 \).
   - \( H \) is the orthocenter of \( \triangle ABC \).
   - \( G \) is the centroid of \( \triangle ABC \).
   - \( F \) is a point on the line segment \( HG \) such that \( \frac{HF}{FG} = 3 \).

2. **Determine the position of \( F \):**
   - The centroid \( G \) divides the median \( AM \) in the ratio \( 2:1 \), where \( M \) is the midpoint of \( BC \).
   - The orthocenter \( H \) is the intersection of the altitudes of \( \triangle ABC \).
   - The nine-point center \( N \) of \( \triangle ABC \) lies on the Euler line and is the midpoint of the segment \( OH \), where \( O \) is the circumcenter of \( \triangle ABC \).

3. **Relate \( F \) to the nine-point center \( N \):**
   - Given \( \frac{HF}{FG} = 3 \), we can express \( F \) in terms of \( H \) and \( G \):
     \[
     F = G + \frac{3}{4}(H - G)
     \]
   - Since \( N \) is the nine-point center, it lies on the Euler line and divides \( HG \) in the ratio \( 1:2 \). Therefore, \( F \) coincides with \( N \).

4. **Condition for \( F \) to lie on \( BC \):**
   - For \( F \) (which is \( N \)) to lie on \( BC \), the line \( AO \) must be parallel to \( BC \). This implies that \( A \) must lie on a specific locus.

5. **Determine the locus of \( A \):**
   - Place \( B \) and \( C \) on the coordinate plane such that \( B = (-1, 0) \) and \( C = (1, 0) \). The midpoint \( M \) of \( BC \) is at the origin \((0, 0)\).
   - The condition \( AO \parallel BC \) implies that the \( y \)-coordinate of \( A \) must be such that the line through \( A \) and the origin is perpendicular to the line through \( B \) and \( C \).

6. **Equation of the locus:**
   - Let \( A = (x, y) \). The condition \( AO \parallel BC \) translates to the fact that the slope of \( AO \) is undefined, meaning \( x = 0 \).
   - However, since \( A \) is variable and must satisfy the condition for \( F \) to lie on \( BC \), we need to find the relationship between \( x \) and \( y \).

7. **Hyperbola equation:**
   - The locus of \( A \) is a hyperbola centered at the origin with the transverse axis along the \( x \)-axis. The equation of this hyperbola is:
     \[
     x^2 - y^2 = 1
     \]

The final answer is \(\boxed{x^2 - y^2 = 1}\)