Answer:
Friday, August 17th
Step-by-step explanation:
Mark plays every 4th day
18th of July,21st of July,24th of July, 27th of July, 30th of July, 2nd of August,5th of August, 8th of August, 11th of August, 14th of August, 17th of August
Bryan plays every 6th day
18th of July, 23th of July, 28th of July, 2nd of August, 7th of August, 12th of August, 17th of August
David plays every seventh day
18th of July,24th of July,30th of July, 5th of August, 11th of August, 17th of August
The next day when all three of them will be at the court together is Friday, August 17th
