Kenko Pro 300/Pro 300 DG 3X is the only 3X AF Teleconverter in the market.
It provides 3 times the magnification of an existing lens.
The effective F-stop is also reduced by a factor of 3.
Example:
50mm F1.7 + 3X = 150mm F4.5
100mm F2 + 3X = 300mm F5.6
200mm F2.8 + 3X = 600mm F8 (EXIF reported F5.6)
Placed between the camera body and lens, a teleconverter contains a set of optics that will effectively increase the focal length of a lens by a specific amount.
The Teleplus PRO 300 converter is made with high quality multicoated optical glass supplied by Hoya Corporation, the world's largest manufacturer of optical glass.
The PRO 300 3.0x can be used with telephoto zoom lenses as well as prime lenses. However, Kenko does not recommend them for lenses that have a zoom range that starts under 50 mm.
KENKO PRO 300 AF Teleplus converters have genuine Gate Array IC (Integrated Circuitry). It means that the converter’s unique circuitry maintain signal integrity between the camera body and lens.
These converters are designed to electronically operate the same way an original manufacturer’s converter would provided there is enough light available.
Important Note:
Due to the 3 stop light loss inherent in a 3x converter, Kenko PRO 300 3.0X has no AFcapabiltiy.
In Reality
Due to the 3X reduction in F stop, it is recommended to use with lens of F2.8 for faster.
Based on my tests,
AF will work with lenses with F2.0 or faster and AF will also work with lenses with F2.8 but only correctly in certain lighting condition.
In dim room, AF will hunt and you may as well switch to MF.
If you are unsure if the will work just send me the camera make and model plus the lens mm and f rating and I will advise