Move timeNowTare = millsmillis(); inside first if:
if (Serial.read() == 't') {
tarestatetareState = true;
timeNowTare = millsmillis();
}
and tareState = false inside second if:
while(tarestate == true(tareState) {
if (millsmillis() - timeNowTare >= 3000) {
Serial.println(F("Tare"));
tareState = false;
}
}