Perhaps the distinction between the one-judge and three-judge court could be made by referring to the former as a "Court" and the latter as a "Full Court", explaining in parentheses the first time you use this term.

https://en.wikipedia.org/wiki/Full_Court

It might be like this.

If you are sure that in your text “juzgado” refers to a “one-judge court”, and “tribunal” refers to a “three-judge court”, you will be safe using these terms.

But this is not necessarily the case.

“Tribunal” and “juzgado” are used interchangeably in Venezuela, and I understand that it is more or less the same in Argentina.

The Diccionario de Ciencias Jurídicas, Políticas y Sociales de Manuel Ossorio (Edit. Heliasta, Buenos Aires, 2004) defines these terms as follows:

Tribunal: Magistrado o conjunto de magistrados que ejercen la función jurisdiccional, sea en el orden civil, en el penal, en el laboral, en el administrativo o en otro fuero, y cualquier que sea su categoría jerárquica.
Se llama unipersonal cuando está constituido por un solo juez, y colegiado cuando lo integran tres o más jueces.

Juzgado: Tribunal de un solo juez
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This Argentinean site also deals with the concepts of “juzgado” and “tribunal”:

La Jurisdicción Penal: (http://www.eco.unlpam.edu.ar/objetos/materias/abogacia/3-ano...
(See “El Tribunal – Concepto” and “El Tribunal – Su Composición”)
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In English, I have seen “one-judge court” for “juzgado/ tribunal unipersonal”, “three-judge court * three-judge panel” for “tribunal colegiado (de tres jueces)”
(http://www.txcourts.gov/media/1243989/December-11-2015-Trans... and “multi-judge court” for courts with more than three judges (such as Supreme Courts, which have 15 or more judges).