Step 1: Calculate the value of Stock A
Number of shares of A = 10, Price = Rs 120.
Value of A = $10 \times 120 = 1200$.

Step 2: Calculate the combined value of B and C
Total Portfolio Value = Value of A + Value of (B + C)
$3300 = 1200 + \text{Value of (B + C)}$
$\text{Value of (B + C)} = 3300 - 1200 = 2100$.

Step 3: Set up equations for B and C
Let the number of shares of stock B be $b$ and stock C be $c$.
1. $b + c = 20$ (Total shares of B and C)
2. $90b + 150c = 2100$ (Total value of B and C shares)

Step 4: Solve for $b$
From equation 1: $c = 20 - b$.
Substitute in equation 2:
$90b + 150(20 - b) = 2100$
$90b + 3000 - 150b = 2100$
$-60b = 2100 - 3000$
$-60b = -900$
$b = \frac{900}{60} = \mathbf{15}$.