mediation in a mixed repeated-measures design

Hello to everyone :)

I write to ask an advice about which type of analysis I should perform in the following situation.

I have 2 groups:
  1. treatment group
  2. placebo group
And two sessions of measurement for each group:
  1. session 1 (pre-treatment)
  2. session 2 (post-treatment)
At session 1 and 2 I measure:
  1. dependent variable
  2. mediator1
  3. mediator2
  4. mediator3
My hypothesis is that the treatment will diminish the dependent variable through influencing the 3 mediators. This is a mediation analysis, but I have a multi-categorical independent variable, plus my measurements are nested into the subjects. How can I perform a mediation in this case?

Thank you very much!