Petechiae rashes on upper chest. What could this be? I have had blood test for lupus and sjogren's syndrome but all came back negative except C3 level

Are they petechiae? Real petechiae do not lose their color when you press down on them. With your dry eyes and joint pain, sjogren and lupus labs are in order. What's your platelet count, did you bleed badly during a dental procedure, and did your physician try something simple like a capillary fragility / tourniquet test? If it really petechiae without signs of clotting trouble, a dermatology consult is in order.