Postgres multicolumn vs combined single column indexes
If the P = '...' AND Q = '...' is highly selective, I would just put one index on that pair of columns and leave it at that.  If it is only moderately selective and if I needed individual indexes on A, B, and C anyway for other queries, then I'd probably tack P and Q onto the end of each of those indexes, unless the P and Q columns are wide and so would bloat the size of the indexes.

I wouldn't use 5 separate single column indexes for this, unless I needed each one for some other purpose anyway.  They would have to be combined with a BitmapAnd, which is much less efficient than using a multi-common index with equality conditions.