This is how I deal with the little fuckers.....
1 - remove infected leaves. Spiter mites can't travel from leave to leave easily until they start making their webs. So carefully remove any leaves with signs of spider mite damage.
2 - Treat plants with Neem oil foilar spray 1ml litre. Spray under leaves as well. You are only 3 weeks into flowering so don't worry about mould (as long as your ventilation is good) and there is plenty of time for the neem to break down. Takes about 10-14 days.
3 - A few days after treating with Neem, release some spider mite predators into your grow room. Available online. They really work well and should win the battle with the reamining mites/eggs.
After your grow is finished, clean and disinfect your room/equipment, then clean it some more. Good luck!