We would advise you to stick with the B3 as this is your final exam in the Level 2 Certificate in Bookkeeping (albeit pre-2019 syllabus). After you have completed all three (B1, B2, B3) you can then transfer onto the new syllabus and take the Level 3 exams: M4, M5, M6, M7 and M8.
Likewise, if someone is part way through Level 3 but on the 'old' pre-2019 syllabus, we would advise them to stick with it until the end.
Please note, the pre-2019 syllabus is being discontiued at the end of the year so only 'new' exams will be available from 2020 onwards. We are more than happy to discuss individual circumstances if you would like to call us on 0203 405 4000.