Allbrother: Mulltclasses get half the HP of the class they're leveling up.
So in Jaheira's case she'll get 1d10 + 3 / 2 (maximum 6) on fighter level-up and 1d8 +
3 / 2 (maximum 5) on a druid level up
The fighter level is half of (d10+3) for a maximum of 6 or 7 depending of if the extra hit point is rounded up or down.
The druid level is half of (d8+3) for a maximum of 5 or 6 depending on if the extra hit point is rounded up or down.
I think the rounding depends on if the total number of levels is even or odd. I have not tested this, so I do not know if it is rounded up or down on odd and even levels, respectively. Here is how I think a roll from level 1/1 to 2/2, or 2/2 to 3/3 works:
The Fighter level gets rolled first. d10+3 yields a number from 4 to 13. Half of the maximum is 6.5. The total number of levels, now 5, is odd and this gets rounded up to 7. Now the Druid level test rolled for d8+3, yielding a number from 4 to 11 and this is divided by 2 rounding down. So a roll of 10 or 11 yields 5 hit points for a total of 12.
Based on this logic, then the probability of rolling 12 hit points for Jaheira is 1 in 40.