Sugarcane . . . . . . . . usually two three meter long?

Sugarcane refers to the plant in general, not individual stalks.
Think of it like this: "Rice is grown in many countries." We're talking about rice as a crop, not individual grains.

Therefore, we need a singular verb.

* "are" and "were" are plural verbs. They'd be used if we were talking about multiple sugarcanes (which isn't the common way we describe sugarcane).
* "was" is singular, but it's in the past tense. The sentence is generally describing sugarcane, so we use the present tense.

"is" is a singular, present tense verb.

So, the complete sentence is: "Sugarcane is usually two three meter long."