Just consider each assertion one by one, and its implications.
- Bob tells the truth.
Therefore Steve is lying and Fred tells the truth => this is possible
- Steve tells the truth.
Therefore Bob is lying and Fred tells the truth => this is possible
- Fred says the truth.
Either Steve or Bob is lying (those possibilities were already evaluated as possible) or both are lying. The latter case is impossible since we would then get that both are not lying, according to their respective assertions.
- Fred is lying.
Then neither Steve nor Bob are lying which is impossible.
Fred tells the truth, for sure, and either Bob or Steve is lying, but not both.
This can be written: Fred AND (Bob XOR Steve)
Note: XOR here is an exclusive OR, contrary to Fred's inclusive OR