If we leave now, we could be there at 3 o'clock.

The outcome is very possible, so it's 1st conditional. Formed by: If + simple present / simple future (will + simple infinitive. Here could can be replaced by many auxiliary modal verbs (will/would/can/could/should/may/might) to indicate how likely it is that they'll get there by 3 o'clock. (will=very likely -- might=unlikely)