To complete the achievement "Finish a jam race three days in a row" you need to cross the finish line in a Jam race at least once a day during 3 consecutive days. For example, finishing a race on Monday, Tuesday and Wednesday will count. If you skip one day then the counter is reset to zero. You can race either Jam track, it can be the same each day or different. As long as it is a Jam track then it counts. Note, the Career pack "Top Jams" will not count toward this achievement. The counter can also reset at some circumstances if you travel to a different time zone or if you change the date and time on your device.