Hi Murali,
1) If you are conversant with TALLY ERP 9, then it should help you in setting up an Accounting Filing System for a company. You can maintain accounts of all three companies simultaneously in the same Tally account. Open Co.A, Co.B, & Co.C, and so on. Start creating required ledger heads and entering the transactions. It's unnecessary to elaborate step by step entries in Tally to someone literate like you.
2) If the Tally account is in operation, there is no need to maintain separate registers; everything can be managed within it.
3) If you are unable to create a separate Assets register in Tally, calculating depreciation is also easy if you create masters like asset description, the rate of depreciation applicable as per laws/acts, etc. Depreciation rates vary depending on the type of assets, usage, expected life of use, etc. The Companies Act/IT Act in India differs from what is applicable in Dubai, so obtain all guidelines and consult competent authorities in Dubai or a CFA if available in practice there.
4) If you use Tally Software, no separate voucher number would be necessary as the software assigns voucher numbers automatically. In Tally, the Profit & Loss Account, Balance Sheet, all linked ledgers, cash book, etc., are also automatically updated as and when you make an entry for every entry. You can access all the books, ledgers, PL, BS together at any time for every entry/date.
If you are struggling to start, consider hands-on training on Tally from a licensed trainer or purchase a Tally guide to help you. Online portals are available for training. There are numerous P&L Accounts and Balance Sheets of various companies on their websites; explore and see for yourself. If you still need the formats, I can provide them. All the best.