+0
Please tell me if these sentences are correct:

1. I'm going to have to call you back later this afternoon. - Said in the morning at 9 AM.

I'm going to have to call you later in the afternoon.

I'm going to have to in the afternoon.

Are all three used correctly?

2. What would you guys want to eat?

What will you guys want to eat?

What's the difference between the two sentences?

3. I had sent the last tender specification which is filled by Paramount (and Eric Lee) with detail measurement! - Can this be a stand alone sentence? Please correct.

Thank you!
+0
abbosI'm going to have to call you back later this afternoon

What does this sentence mean exectly?
It will be necessary for me to call you back later this afternoon because I can't talk to you now. (The tone is somewhat apologetic. It is too blunt to say I have to call you back later or I'm going to call you back later.)

CJ
Comments  
1. I'm going to have to call you back later this afternoon. - Correct

I'm going to have to call you later in the afternoon. - Correct

I'm going to have to in the afternoon. - You need at least a verb after "have to" in this sentence. For example, "I'm going to have to go (verb) to the dentist's in the afternoon" or "I'm going to have to wash (verb) my hair."

Are all three used correctly?

2. What would you guys want to eat? - I think it is acceptable grammar if you are speaking hypothetically. For instance, "What would you guys want to eat if you were to go to the restaurant?" If you are not speaking hypothetically I don't think it's correct grammar.

What will you guys want to eat? - Acceptable grammar if you are speaking about the future. For instance, "What will you guys want to eat when you come over later this evening?"

What's the difference between the two sentences?

3. I had sent the last tender specification which is filled by Paramount (and Eric Lee) with detail measurement! - Can this be a stand alone sentence? Please correct.

This last sentence makes absolutely no sense at all. Perhaps if you changed some of the words and reworded it or gave more context it would make sense.
Teachers: We supply a list of EFL job vacancies
I'm going to have to call you back later this afternoon

What does this sentence mean exectly? as to me it's not clear. Please explain the meaning of this sentence. 

Maybe "I'm going to call you back later this afternoon" more understandable? Or may be he'd better say: "I want to have your call-back later this afternoon" It means that He expects his call-back later this afternoon.
Anyway I didn't understand exact meaning from above-written sentence. 
 CalifJim's reply was promoted to an answer.
Thanks CalifJim!
Students: We have free audio pronunciation exercises.
3. I had sent the last tender specification which is filled by Paramount (and Eric Lee) with detail measurement!

had + Vpp is past perfect tense..you should have written this way "i sent the last......." (past tense) because past perfect is used only if you have two past action/event (past action before another past action) i.e. i had been drunk when you arrived.