Is it obligatory for a girl who suffers from excessive discharge to perform ablution for every prayer (obligatory and supererogatory), and is it permissible for her to combine Dhuhr and Asr prayers with a single ablution?

If the discharge you see is vaginal moisture and it exits continuously such that you do not find sufficient time to pray with proper purification, then your ruling is that of one afflicted with سلس (urinary incontinence/continuous discharge). Perform ablution for each prayer after its time has begun, and pray the obligatory prayer and as many supererogatory prayers as you wish until the time expires. It is permissible for one afflicted with سلس to combine two prayers during the time of one of them, and there is no harm in adopting this opinion when there is a need.