Is it best to use only 1 troop type or a mix of all 3 when attacking or defending?

For a march when attacking this will depend on the heroes that you can use and the levels of investments you have made into each type of troops ATK / DEF / HP bonuses. Some heroes give bonuses to fixed troop types, while other will give general troop bonuses.

Have a look at your strongest heroes and see what Troop skills they provide.

Then check your stats from your gear / chips / modules etc. If you have better stats for Fighters for example and you have heroes with good fighter stats then attacking with fighter based heroes and full fighter squad will be much more powerful. However if your stats / heroes are more general then you may find that a mixed troop squad will work best as they can compliment each other.

In the long run it may be better to focus on one troop type (upgrade that types heroes and focus on upgrading stats for it).