A de Bruijn Sequence, as defined in N.G. de Bruijn's A combinatorial problem, Proc. K. Ned. Akad. Wet., vol. 49, pp 758-764, 1946 (with attribution to Ir. K. Posthumus) is
an ordered cycle of $2^n$ digits 0 or 1, such that the $2^n$ possible ordered sets of $n$ consecutive digits of that cycle are all different.
The example given for $n=3$ is the ...
Clocked is an electronics term and here it applies to the LFSR's implementations;
Assume that the LFSRs are implemented with D-FLip-Flop that can store values in bits.
At each clock, the waiting input (D) passes the output (Q) and in LFSRs that go to the next cell. This is the shift operation.
The clock of the system simultaneously acts on D-fLip-flops ...
The question as stated has some serious issues:
The alternating step generator works with 3 LFSRs. If the output of R1 is 1, then R3 is clocked, otherwise R2 is clocked, so at each clock period exactly one of those two is clocked. The output of R2 and R3 is added modulo 2 to form the keystream. The initial loadings of the 3 LFSRs is essentially the key.