Three crack heads can, if their story is believed, provide evidence sufficient to convict a person. While some evidence tended to exonerate other evidence tended to implicate. This is only based on what you posted.
Use a lawyer to assist prior to sentencing.
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It is up to a jury to decide what evidence to believe or not believe, and how much weight to give each bit of evidence. If the jury believed the evidence against your husband, then they could find him guilty of the offense. If the judge agrees with your version of the evidence, you might find that he gives your husband a lighter sentence than expected. Follow the advice of your husband's attorney and cooperate with him in any matter in an effort to help your husband.
It was not a situation in which your husband was convicted "without any evidence." As you say, your husband was implicated as the shooter by three witnesses. Regardless what you may feel about their character and by implication their credibility, whether or not to believe the testimony of those witnesses is up to the jury, or if this was a bench trial, the judge.