I have the following LineLegend:
       
    Labeled[
     Plot[Exp[-0.03*t], {t, 0, 100}, Frame -> True]
     , LineLegend[{Blue, Green, Red}
      , {"a = test1
        a = test2"
       , "a = test3 
        a = test4", "a = test5 
        a = test6"}
      , LegendLayout -> "Row"
      , LegendMarkers -> {
        {"\[FilledCircle]", 15}, {"\[FilledUpTriangle]", 
         15}, {"\[FilledSquare]", 15}
        }
      ]
     ]

I tried to stack, e.g., a = test1 and a = test2 in a column but keep the overall legend presented as a row by pressing the Enter key between a = test1 and a = test2, but not only did it not work, it seemed like a hacky way of solving the problem. 

Is there a way to stack legend item text while maintaining a "Row" layout?