No, the above is not a typo. Typically, the Christian doctrine of the Trinity is target number one for other monotheists seeking to refute Christianity. They accuse us of tri-theism or, at best, equivocation / ambiguous terms.
Simply put, tri-theism is a strawman (we don't teach that). Incidently, that creates an argument against Islam since the Koran makes this a cornerstone of its disagreement with Christianity. Since it seems logical to assume that God understands Christianity better than me, and I understand that Christians are monotheists, but the Koran does not grasp this fact, then the Koran is not from God (and Islam would be, correspondingly, false).
The accusation of equivocation or ambiguity is, from a philosophical point of view, stronger, but it is refutable through a strict rejection of pure philosophy in favor of the Orthodox apophatic approach (and indeed, the Cappadocian theologians who articulated the doctrine of the Trinity made a point to state it apophatically - that God is beyond number, beyond arithmatic, such that concepts like 3 and 1 can't fully apply).
Yet the doctrine of the Trinity can indeed form a core argument FOR Christianity. More after the jump: