How can I make my husband legal?

If he entered the country legally, he can apply for adjustment of status. The fact that you have three kids together and you have not already worked on adjustment of status papers makes me believe that he entered illegally and is not adjustable under current law. He may be able to apply for permanent residence under the I-601A program under which you would petition for him on the I-130 petition, have it approved, and begin consular processing which includes obtaining proof of fee receipts from the National Visa Center of the Department of State. He would then be eligible to apply for the I – 601A waiver of illegal stay with USCIS with the criteria being that the waiver would be granted if extreme hardship were to occur to you if you were separated by the 10 years illegal stay bar. If the waiver is granted, he could then process the rest of his immigrant visa case with the American consulate or embassy with jurisdiction over immigrant visa issuance in his own country. The appeal of this particular waiver is that your husband would be able to see whether it is approved or not before making a decision to leave the country for an immigrant visa appointment. Please note however that this waiver is only available where your husband his only inadmissible through his illegal stay in the country.  The I-601A waiver does not excuse other grounds of inadmissibility such as fraud or crimes. An alternative may be to wait and see what will happen with comprehensive immigration reform this year. Due to the limitations of the Lawyers.com Forums, Alan Lee, Esq.'s (the "Firm") participation in responding to questions posted herein does not constitute legal advice, nor legal representation of the person or entity posting a question. No Attorney/Client relationship is or shall be construed to be created hereby. The information provided herein by the Firm is general, and requires that the poster obtain specific legal advice from an attorney. The poster shall not rely upon the information provided herein as legal advice nor as the basis for making any decisions of legal consequence.

If your husband has no other immigration issues other than having been here without authorization for so long, you can petition for him  (I-130) and then he can file for the provisional waiver (I-601A) from here in the US.  He'll have to go home to get and immigrant visa once the waiver is approved, but that process shouldn't take long and he'll return with a green card if he qualifies for the waiver.