Modal verbs in 'If clause' in conditionals
Let's examine what first conditional and second conditional mean.-
Ref. British Council website
https://learnenglish.britishcouncil.org/grammar/b1-b2-grammar/conditionals-zero-first-second#
We use the first conditional when we talk about future situations we believe are real or possible.
In first conditional sentences, the structure is usually: if/when + present simple >> will + infinitive.
It is also common to use this structure with unless, as long as, as soon as or in case instead of if.
The second conditional is used to imagine present or future situations that are impossible or unlikely in reality.
The structure is usually: if + past simple >> + would + infinitive.
When if is followed by the verb be, it is grammatically correct to say if I were, if he were, if she were and if it were. However, it is also common to hear these structures with was, especially in the he/she form.
Modal Verbs in 'if clause'
If' can be followed by a modal verb. Adding 'will' or 'would' after 'if' conveys an agreement. . For example:
If you will pick him up, I will be grateful = If you agree to pick him up...
It can be used to highlight that someone's consent is necessary or as an especially polite form.
'If can/could --- also has a similar use. It is a polite way of saying, "If it is/were possible---"
Now let's look at OP's three sentences :-
(1) If you can help me understand the word "try" as you used it, that would be very helpful to my understanding.
Yes, this is a mixed conditional sentence between type 1 and 2 with a polite 'if clause'.
(2) If you can help me understand the word "try" as you used it, that will be very helpful to my understanding.
This is a 1st conditional sentence with a polite 'if clause'.
(3) If you could help me understand the word "try" as you used it, that would be very helpful to my understanding.
No, this is not a 2nd conditional sentence, but a mixed conditional like sentence 1. 'could help' is not simple past but a polite request. It is more polite than 'can help'.
Ref. https://ludwig.guru/s/if+you+could+please#:~:text=You%20can%20use%20this%20phrase,%2C%20I%20would%20appreciate%20it.%22&text=Feinstein%3A%20%22If%20you%20could%20please%20expedite%20the%20removal%22.&text=If%20you%20could%20please%20step%20out%20of%20the%20truck.
