You are probably calling it a “triathalon” when it is actually a “triathlon”
Here we have another case of the mystery additional letter. For some reason, every one likes to call this super-athletic event a “tri-ath-a-lon” when, really, it is pronounced as it is written.
On a side note, have you noticed how most of these commonly mispronounced words are healthy foods or forms exercise. Hmmmm…