Can I use change/difference score as DV in regression?

Dear members,

I'd like to run a mediation analysis in R with the following data:
"treatment" ("a","b","c") as predictor
"expectancy" of relief as mediator
"symptom intensity" as outcome variable

symptom intensity was measured at 3 times
- t0: pre random allocation to the treatment groups as baseline
- t1: midpoint
- t2: postintervention

I expect treatment "a" to be superior to "b" and "c" in reducing symptom intensity. And I also expect that this effect is mediated by expectancy of relief.

Now, I have problems calculating the regressions for the mediation analysis:
For the predictor I have to code dummy variables I guess, right? But I'm not sure how to handle the outcome variable as I have longitudinal data. I thought of calculating a change score (t2-t0) and use it as the dependent variable. But I didn't find any papers really supporting this method even being bad as it leads to regression to the mean .. does anybody have a solution for me how I can handle the longitudinal outcome?

thank you very much in advance!!