Divine Smite can be used when the player makes a successful melee attack and at level 11 improved divine smite automatically adds 1d8 damage to all melee attacks. When wielding the tentacle rod you use your action to direct each of 3 tentacles to attack at +9 that do 1d6 damage.
Can I use Divine Smite for all 3 of these attacks? For any of them?
Best Answer
No
The rules
The main reason for this is that the Tentacle Rod makes melee attacks, while Divine Smite requires melee weapon attacks.
So melee attacks are considered melee weapon attacks only when made with a handheld weapon, otherwise they are plain melee attacks (coincidentally, tentacles are actually given as an example of this case).
The difference between the different types of attacks is very well explained by Dale M in his answer to a different question, but to reiterate:
Additional musings
Given that the attack is made by the tentacle itself, while the PC uses his/her action to use (command) the rod (which isn't a function of the Use an object action - DMG p. 141) and not take the Attack action, this could also be argued that disqualifies the attacks for the Divine Smite requirement, as you have to be the one making the attacks: