I heard that for new volunteers the Thai government started giving visas for only 3 months each time - Has anyone heard about it? Is it correct? 

Thanks in advance. 

You would get a single entry visa and then extend in country.

Since the new police order went into effect a little over 2 years ago some extensions (it is not a visa) based upon working as a volunteer are only for 90 days at a time and will only be issued to give a total stay of one year. After one year a new non-o visa must be applied for at an embassy or consulate to start a new year.

From police order 327/2557. 

Quote

2.21 In the case of necessity where the alien must stay to perform duties in a public charitable organization, a foreign private company, a foundation, an association, a foreign chamber of commerce, the Thai Chamber of Commerce, or the Federation of Thai Industries:
(1) Each permission shall be granted for no more than one year.
(2) In the case where there is no certification made by any government agency under Criteria (3) hereof, each permission shall be granted for no more than 90 days of stay, the total period of stay shall not exceed one year from the date of entering the Kingdom.
The alien:
(1) Must have been granted a non-immigrant visa (NON-IM).
(2) Must have been confirmed and requested by the authorized person or the person holding the highest position in the organization, and/or
(3) Must have been confirmed and requested by the head of a government agency equivalent to or higher than the departmental level or by the head of a state enterprise or by the head of another government agency relating to the organization.