I am not Sure if I understood you correctly but based on my understanding I took the sample dataset which comes with tableau (SuperStore)and created individual calculations for each segment
For Consumer Segment
IF ATTR([Segment])="Consumer" THEN (SUM([Sales])/TOTAL(SUM([Sales])))*100
That gave me % of Consumer
Similarly the remaining two such as "Corporate" and "HomeOffice"
Create another calculation Sumpercentages where it adds Consumer+HomeOffice+Corporate
Kindly Let me know if this is not what you are looking for