Here they are asking for a guaranteed deadlock, and we can only guarantee a deadlock when we have fewer resources than the max requirement by any candidate.
Lets label these 3 processes, $p_1,p_2,p_3$,
$R_{max-guaranteed-deadlock} = max(R_1, R_2,R_3) -1$; where $R_i$ is the requirement of $i^{th}$ process.
$\implies R_{max-guaranteed-deadlock} =2 -1 = \mathbf{1} $.
Note that the maximum value for which we may have a deadlock is different from the max value to guarantee a deadlock.
Max possible value for which we may have deadlock = $\displaystyle \sum_{i=1}^{3}(R_i -1) = 3$
The minimum value to guarantee no deadlock= Max possible value for which we may have deadlock + 1 = 3+1 = 4.
Word guarantee changes everything.
