Step 1: The total area of the plot is:
\[
{Total area} = 20 \times 15 = 300 \, {m}^2
\]
Step 2: The ground coverage is 50%, so the area covered by the building on the ground is:
\[
{Ground coverage} = 0.5 \times 300 = 150 \, {m}^2
\]
Step 3: The Floor Area Ratio (FAR) is 3.0, so the total floor area that can be constructed is:
\[
{Total floor area} = {FAR} \times {Total area} = 3.0 \times 300 = 900 \, {m}^2
\]
Step 4: The number of floors that can be constructed is the total floor area divided by the ground coverage area:
\[
{Number of floors} = \frac{{Total floor area}}{{Ground coverage}} = \frac{900}{150} = 6
\]
Conclusion: The maximum number of floors that can be built on the plot is 6.