How big were those three screens?
I add on screen repair (mesh replacement), depending on size for $10, $20, and $35 for screen doors. Be aware that solely doing a "Rescreening Service" would probably require you to focus on a different pricing structure?
NOTE: I stated an ADD ON SERVICE, which means in addition to what ever other service they are buying.
If the corners are brittle then the frames are usually corroded and soon to be shot too - I live on a peninsula with salt water on all sides, so new screens and frames last about 5(?) - 7 years depending on exposure and frequency of cleaning.
Generally speaking for pricing you should run it about 2.5 times cost. So lets say it costs you $4.66 to buy material (3' x 3' mesh from a 25' roll $1.80, 12' of spline from a 100' roll 96¢, 4 plastic corners $1.90 = $4.66 ), then you should sell it for $11.65. (Just a sample formula, you have to pay tax on that too! Just focus on a formula for now). Have a minimum for your business too.
If the cost for one or two screens comes out to like $4 - $5, is $10 or $12 worth it to you? Set a minimum to build or re-mesh one screen. Time is money and you have dreams to chase! If the customer has 15 screens that need to be redone, then your minimum covers your time and cost to stay in business. Profit is not a hobby.