Is there such thing as a type III muscle fibre?

Recently I have come across a few sources that have stated the existence of a type III muscle fibre. The theory is that by subjecting the body to different forms of physical exercise ranging from high and low intensity cardio to high and low intensity strength training in the same workouts, you stimulate the growth of all different muscle fibres. Over time the muscle "morph" together to form a hybrid fibre which is able to sustain large amounts of work over a long duration. The background information is based on the training and demands of ancient warriors such as the Gladiators, Spartans and Vikings. The Vikings for example would have to row for days on end across rough seas then beach their longboats and proceed to carry them over rough terrain while also transporting supplies with full gear. After this they would then have to fight a battle and then continue trekking, etc. Here are some of the sources:

youtube-strengthcamp

len shwartz

ori hofmekler

I am unsure if this is true but if there is any scientific evidence that people could put forward, it would be appreciated.