If all three of them participated in the robbery and one of them displayed a gun during the robbery, then all three are guilty of the aggravated robbery. Texas Penal Code sections 7.01 and 7.02 state:
Sec. 7.01. PARTIES TO OFFENSES. (a) A person is criminally responsible as a party to an offense if the offense is committed by his own conduct, by the conduct of another for which he is criminally responsible, or by both.
(b) Each party to an offense may be charged with commission of the offense.
(c) All traditional distinctions between accomplices and principals are abolished by this section, and each party to an offense may be charged and convicted without alleging that he acted as a principal or accomplice.
Sec. 7.02. CRIMINAL RESPONSIBILITY FOR CONDUCT OF ANOTHER. (a) A person is criminally responsible for an offense committed by the conduct of another if:
(1) acting with the kind of culpability required for the offense, he causes or aids an innocent or nonresponsible person to engage in conduct prohibited by the definition of the offense;
(2) acting with intent to promote or assist the commission of the offense, he solicits, encourages, directs, aids, or attempts to aid the other person to commit the offense; or
(3) having a legal duty to prevent commission of the offense and acting with intent to promote or assist its commission, he fails to make a reasonable effort to prevent commission of the offense.
(b) If, in the attempt to carry out a conspiracy to commit one felony, another felony is committed by one of the conspirators, all conspirators are guilty of the felony actually committed, though having no intent to commit it, if the offense was committed in furtherance of the unlawful purpose and was one that should have been anticipated as a result of the carrying out of the conspiracy.
As you can see from § 7.02, all the joint participants in a robbery would be responsible for the acts of the other participants that occur during the commission of the crime, and as a result under § 7.01 all of them may be charged with the crimes that any one of them committed. So, if one of them displays a gun during the robbery and thus commits an aggravated robbery then ALL of them may be properly charged with and convicted of the aggravated robbery. If the sentence he was given was within the allowable range for an aggravated robbery he’s unlikely to win an appeal of the conviction or sentence if the only argument here is that he should not have been convicted of aggravated robbery because it was the other guy who had the gun, not him.
Bear in mind that there is a very short time frame for appeals and he only gets one shot at it. So he really needs a lawyer to assist him if he has any viable appeals issue to pursue.