It will be 7.
For a B+ tree,
Minimum number of keys = Minimum number of block pointers - 1
And minimum number of block pointers for a B+ tree in any non root node is given as $\left \lceil \frac{p}{2} \right \rceil$ where p is the maximum number of block pointers. For a root node, minimum number of block pointers is 2 as minimum 1 key value must be present
For your question, minimum number of block pointers = 3 + 1 = 4
that gives $\left \lceil \frac{p}{2} \right \rceil$ = 4 , So p = 8.
So, maximum number of keys = 8 - 1 = 7