How are you gonna control military governments in Central America? These guys aren’t giving up their power and control for a few billion, imo.
They're generally not military governments and the USA is already dealing in aid to all of those countries, the problem being that they are currently engaged in the kind of aid (primarily military/security) that makes the problems WORSE, not better.
Guatemala is a democracy (though a somewhat unstable one). We gave them $250 million in aid last year, but the biggest chunk of that was in counternarcotics programs that probably only make things worse. If we bumped that up to half a billion, cut out the narcotics programs, and focused on agricultural/education assistance, we could make a big difference there. In fact, the BIGGEST difference would be if we made say a one-time $1 billion payment conditional on land reform. Poor people in Guatemala just don't have enough land to make a living on, land seizures by wealthy agribusiness is the reason why. Those people have the ear of government because they have the money, but it would be easy for the USA to shift that ear, especially if they dropped their own corporate involvement in the problem.
El Salvador is also a democracy, though their government is somewhat weak and corrupt and MS-13 moves freely. The justice system is particularly bad, murders are rarely solved. We gave them about $120 million in aid in 2017, bump that up to $300 million focused on economic improvements and modernization of the justice system, dependent on an anti-corruption program. Again, while many politicians are corrupt, money talks to them, and the USA has more money to throw around the country than anyone there has. We did a lot to prop up El Savador's worst governments and create this problem, we can work to help them build a better one instead.
We gave Honduras about $180 million in aid in 2017, mostly to prop up governance. Honduras is ruled by an unelected leader propped up by the legislature after a 2010 coup. $20 million of our aid is directly to the military that initiated the coup. If we dropped all military/security assistance, but offered to triple our give to $500 million/year dependent on free elections, there's a good chance that the coalition allowing the coup would crumble. It wasn't a hard coup, they rely on the support of the legislature, and even they can recognize that if they fell back to hold only military power in a more economically advanced democratic state, they're cutting their losses just fine. Even if that pipe dream doesn't happen, the USA can at least drop the military aid and focus solely on agricultural/education/humanitarian aid.
We're already dumping money in all these countries, we're just doing it in a way that serves some corporate interests and extends the War on Drugs. If we instead focused on things that directly improve the lives of the people there, we'd have enough money to shift governments significantly in the direction we would want (as we already do).
And lets pretend a worst-case scenario, that we can't do anything in any of those three countries. Then instead, offer Mexico $500 million a year in economic and development aid in southernmost Mexico in exchange for helping Central American refugees to resettle there. Mexico has already been asking for that for some time, although at much lower levels. We currently are throwing about $300 million a year to Mexico but it's all drug war shyt. People don't like to go far from home if they can help it, if we helped make southern Mexico a more viable legal destination for refugees, significantly fewer of them would push all the way up to the USA only to risk deportation and re-deportation.