Hi Surya,
Meanwhile you get response from Gopi, I would like to notify you that all these three are equally in demand however something which is quite niche or difficult to fulfill the capacity is TPM (Trade promotion management) which is more towards marketing. If you think from business perspective, marketing is quite important and almost implemented everywhere. Sales and services are majorly used by companies which has well established customer base and continously putting efforts in retaining and improving customer satisfaction.
So, if you get an opportunity to learn or work within TPM under marketing, it will be value addition to your profile.
Regards
Shanu.