Dear altruists,
I have an independent variable "tob_smk", dependent variable "multimbd_cat" and "phy_activity" is one of the co-variates.

PHP Code:
*tab tob_smk

Ever smoked |
 tobacco on |
    regular |
     basis, ||      Freq.     Percent        Cum.
------------+-----------------------------------
      Never |      3,282       84.24       84.24
    Current |         27        0.69       84.93
     Former |        587       15.07      100.00
------------+-----------------------------------
      Total |      3,896      100.00

 *tab multimbd_cat

  Categories |
          of |
multimorbidi |
          ty |      Freq.     Percent        Cum.
-------------+-----------------------------------
          No |      2,342       60.11       60.11
         One |      1,160       29.77       89.89
Two and more |        394       10.11      100.00
-------------+-----------------------------------
       Total |      3,896      100.00

*tab phy_activity

 Physical activity |
    level category |      Freq.     Percent        Cum.
-------------------+-----------------------------------
Extremely Inactive |        532       13.66       13.66
         Sedentary |      2,125       54.54       68.20
 Moderately Active |      1,029       26.41       94.61
 Vigorously Active |        210        5.39      100.00
-------------------+-----------------------------------
             Total |      3,896      100.00 
Now, my supervisor asked me to see if physical activity act as a mediator between the association of tobacco smoking and multimorbidity. I tried to find the answer everywhere, but could not reach a conclusion. Any help? Which command I need to use or how to do that?