I am 74 years old and I run 3 miles in 29 minutesl 3 to 4 times a week. The other days I lift weights or play singles tennis. I feel fine. Too much?

Keep it up! Exercise has many benefits including heart and bone health, as well as for your mental health. I commend you for your routine. If you have been doing that consistently without problems, keep it up! no reason to change.
Ok if no symptoms. Running 3 miles 3 times a week, lifting weights, and playing tennis are fine to do as long as there are no muscles/joints symptoms, or heart/respiration symptoms, during or after the activities. One should be sure to have a thorough and detailed primary care doctor, who can take care the common problems that can sneak up on people over age 70, and refer to specialists for recommended screenings.