Labeled[Plot[Exp[-0.03*t], {t, 0, 100}, Frame -> True]
     , LineLegend[ {Blue, Green, Red},
      {"a = test1\na = test2"
       , "a = test3\na = test4"
       , "a = test5\na = test6"}
      , LegendLayout -> {"Row", 1}
      , LegendMarkers -> {{"\[FilledCircle]", 15}, {"\[FilledUpTriangle]",
          15}, {"\[FilledSquare]", 15}}]
     ]

[![enter image description here][1]][1]


  [1]: https://i.sstatic.net/EwsLV.png