We took a shuttle through the Bigfoot hostel. It goes from Antigua to Leon. But not that long ago there was a report of a robbery. I suppose you could ask Bigfoot about that.
Or look at Transporte del Sol. They go from Guatemala to Nicaragua without an overnight in San Salvador.
I have not been to El Salvador but I would not hesitate going. I was just not sure if you actually wanted to visit El Salvador or you were just trying to get to Nicaragua. And the armed robbery apparently happened in April when the shuttle was still in Guatemala.
If you want to visit El Salvador I would go to San Salvador (take Ticabus or Transporte del Sol). I believe Suchitoto is not far from San Salvador so you should be able to take a bus there.
To get to Nicaragua I know you could go via La Union. But only a few times a week. I am sure people have arranged it all on their own as well but there is also this company which makes it easier:
You would end up in Potosi, Nicaragua. From there you can hike up volcano Cosiguina. And then you can take a bus to Leon but you could stop in Jiquilillo on the way.
But three countries with only 25 days seems rushed to me. You could easily spend 25 days in Guatemala. Or in Nicaragua. If you combine those two maybe 15 days Guatemala and 10 days Nicaragua.
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