Step 1: Write the equation of the required plane.
The required plane is parallel to
\[
3x+4y-5z=0
\]
So, its equation will be
\[
3x+4y-5z+d=0
\]
Step 2: Use the point \((1,2,3)\).
Since the plane passes through \((1,2,3)\), we substitute:
\[
3(1)+4(2)-5(3)+d=0
\]
\[
3+8-15+d=0
\]
\[
-4+d=0
\]
\[
d=4
\]
Hence, the plane is
\[
3x+4y-5z+4=0
\]
Step 3: Find intercepts on coordinate axes.
For \(X\)-intercept, put \(y=0,z=0\):
\[
3x+4=0
\]
\[
x=-\frac{4}{3}
\]
So,
\[
a=-\frac{4}{3}
\]
For \(Y\)-intercept, put \(x=0,z=0\):
\[
4y+4=0
\]
\[
y=-1
\]
So,
\[
b=-1
\]
For \(Z\)-intercept, put \(x=0,y=0\):
\[
-5z+4=0
\]
\[
z=\frac{4}{5}
\]
So,
\[
c=\frac{4}{5}
\]
Step 4: Find \(3a+b+5c\).
\[
3a+b+5c
=
3\left(-\frac{4}{3}\right)+(-1)+5\left(\frac{4}{5}\right)
\]
\[
=-4-1+4
\]
\[
=-1
\]
Step 5: Final conclusion.
Therefore,
\[
\boxed{-1}
\]