What is the least possible number of planks of timber 42 m, 49 m and 63 m long have to be divided into planks of the same length?

The bells toll after 7, 8, 11, and 12 seconds.
First, find the prime factorization of each number:
$7 = 7^1$
$8 = 2^3$
$11 = 11^1$
$12 = 2^2 \times 3^1$

The LCM is the product of the highest powers of all prime factors present:

This means the bells will toll together every 1848 seconds.

The question asks about the next 3 hours.
1 hour = 60 minutes
1 minute = 60 seconds
1 hour = $60 \times 60 = 3600$ seconds
3 hours = $3 \times 3600 = 10800$ seconds

The bells toll together at 9:00 a.m. We need to find how many times they toll together after this initial time within the 3-hour period.
Divide the total time in seconds by the LCM:

Since we are looking for the number of complete tolling cycles within the 3 hours, we take the floor of this value.
The bells will toll together 5 more times within the next 3 hours.

The final answer is $\text{5}$.