26. Is it true the SDA Church really teaches Tritheism?

No. The SDA Church fully endorses the ‘orthodox’ notion of the Trinity – it is fundamental belief #2:

“There is one God: Father, Son, and Holy Spirit, a unity of three co-eternal Persons.”

See: http://www.adventist.org/beliefs/fundamental/index.html

For a further official exposition of the Trinity from the official SDA theological website, see:

http://www.adventistbiblicalresearch.org/documents/Christ.htm

This allegation is often raised because the early history of the SDA Church saw a progression from a predominantly Arian stance (denying the full deity of Jesus) to a more orthodox belief in the Trinity.

See:http://www.adventistbiblicalresearch.org/documents/trinitydoc%20among%20sda.pdf

One might also be confused with various SDA offshoots, such as the Seventh-day Adventist Reform Movement (SDARM), who do hold a belief of the Godhead which is essentially Tritheism:
       See: http://www.sevytaliban.blogspot.com.au/p/godhead.html
Furthermore and to be fair, some commentators suggest all of Western Christianity, especially English-speaking Christians, do not possess a proper understanding of the Trinity, but instead profess a type of semi-Tritheism.  This is because the English translation of the Trinitarian formula, ‘One God in Three Persons’, is an imperfect one.  As Roger E Olson notes in The Story of Christian Theology: Twenty Centuries of Tradition & Reform (1999), the ancient Fathers did not mean ‘person’ as understood in English today – i.e. an individual, self-actualized center of free will and conscious activity.  Rather, ‘person’ is a poor translation of the Latin ‘persona’, which in itself is arguably a poor translation of the original Greek ‘hypostasis’. 

For a Roman Catholic perspective on ‘person’, see:

http://oce.catholic.com/index.php?title=Person

For an Eastern Orthodox view, see:

http://www.oca.org/OCIndex-TOC.asp?SID=2&book=Doctrine&section=The%20Holy%20Trinity

For an official SDA perspective from the General Conference’s Biblical Research Institute, see:

http://www.adventistbiblicalresearch.org/documents/Doctrine%20of%20the%20Trinity.pdf