Derm noted 3 moles on back. All 3mm or under. One shade and pretty much round or oval. He said slightly dysplastic so best to remove now. What does that mean. .. Are they likely to be melanoma?

Never know. No one know for sure until a biopsy comes back, which provides definitive diagnosis. Melanoma often tends to be very dark. It sounds like you may have some "basic" lesions that just need to be evaluated further as recommended by your derm. Best wishes.
Signs. Aad malignant melanoma: asymmetry: 1 half is unlike the other. Border: irregular, scalloped or poorly defined. Color: varied with shades of tan, brown or black; or white, red or blue. Diameter: > 6 mm. Evolving: mole /skin lesion looks different from the rest or is changing in size, shape or color. An irregular mole that itches, bleeds or is painful is worrisome for possible cancer.