Generally most doctors consider one miscarriage bad luck, the second tragic and the third establishes a pattern. Because your first miscarriage is so far back and you had a live birth after, it will not be considered for diagnosis at this point.
If your doctor has not suggested any testing for you, see a different doctor. After 3 miscarriages it is time to see a fertility specialist.
Miscarriages can have any number of reasons: hormonal and genetic factors are the most common, and they are mostly influenced by maternal age (you do not write how old you are) and health factors (but paternal factors are also involved occasionally).
Usually workups start with blood tests (hormone levels), a thorough gynecological exam and ultrasound to determine any structural issues of the uterus.
You don't seem to have trouble achieving a pregnancy, just keeping it, so your doctor may suggest you try again and then monitor your hormone levels very carefully. If a hormonal cause is suspected or diagnosed you can be prescribed hormone therapy to help sustain the pregnancy.
Please keep in mind that if you are older (35 or even 40+) the quality of the genetic material is declining and there is no treatment available for this other than relying on an egg donor.
Good luck!