Great. So, the first year of the Extended Diploma actually earns you a Foundation Diploma, so we can work out the grade for that. Although the Extended Diploma's grade is three letters (e.g. MMP) the Foundation Diploma's is a single letter (e.g. M). We don't really care about the grade, it's the
points we're after. (Each unit earns you points, and it's the sum of the points which gives you your overall grade.)
You've mentioned two scenarios above, where you either get an M or a D in Unit 3. External (exam-based) units actually allocate you points based upon a sliding scale. For example, in Unit 3 an M could mean anything between 20 and 31 points (depending on your mark), with a D being 32 points. The same is true in Unit 1, where you have a P. That could be anything between 9 and 14 points. As a result, we're going to have to treat your overall points for this year as a
range.
At the bottom end, where you just scrape a P for Unit 1 and just scrape an M for Unit 3, you have a total of
99 points for an overall
M this year:

At the top end, where you're
just short of an M for Unit 1 (but still get a P), and get a D for Unit 3, you have a total of
116 points for a
D overall this year:

The first year of a Foundation Diploma is 510 guided learning hours (GLH), whilst the full two-year Extended Diploma is 1080 GLH. We can use these numbers to scale-up your current points (somewhere between 99 and 116) to what you
might get for both years. So that's between 99 x (1080 / 510) and 116 x (1080 / 510), or between 210 and 246 points. The relevant points thresholds for Extended Diploma grades are below:

So
you're most likely to achieve DDD overall, but it could be a grade lower (DDM) or a grade higher (D*DD) than that. This is assuming you continue working at the same level as you have been.