A Technical Caveat
The percentages are taken straight from the "genetic engine" scripts, but they're really only approximate. That's because the Linden Scripting Language (LSL) -- and many other programming languages -- uses "pseudo random numbers" rather than truly random numbers. Moreover, LSL generates random numbers in batches, which means that fetching several random numbers at about the same time on the same sim may not be truly independent events. At any rate, it's close enough that the "genetic odds" in the table aren't too far off, and the unpredictability adds a little fun (and uncertainty) to breeding!
Calculating Odds for Multiple Traits
Leaving aside the possibility that "random" events in LSL may not be truly independent, the odds of independent events happening together are multiplicative. Thus, the odds of germinating a seed and getting a 3-seed dwarf female would be 2.3% (odds of 3-seed female) times 2.0% (odds of dwarf) for a 0.046% chance (less than one in 2,200 odds). That's kind of uncommon!
Beating the Odds
Obviously, one way to get those 3-seed dwarf females is to germinate lots & lots & lots & lot of ḈḁṇṇḁḇḭṦḼ seeds. Naturally, the ḈḁṇṇḁḇḭṦḼ Team encourages this approach, because it puts more money in our pocket! But most experienced growers take the long view, isolating dwarf genetic lines, taking a chance with Plant Auxin & waiting for that 3-seed female to hit! Happy Growing!