Yes. You can claim exemption under all these sections together, but you have to exhaust exemption under section 54 first, since it doesn’t have a ceiling limit which is present in the other two.
Salary paid by a foreign Government to its employees serving in India is taxable under the head "Salaries" u/s 15 of the 1961 Act. The words ‘an employer’ occurring in clause (a) of that section are wide enough to include a foreign Government. But if ...
Salary accrues where the services are rendered even if it is paid outside India; Hence, the income received outside India against services rendered in India will be taxable in India. (Sec 15 & 9, Income Tax Act)