Writing idiomatic Elixir code

You can do that by using if as an expression and assembling list of values finally concatenating them together:

IO.iodata_to_binary([
  if(data["a"] > 0, do: "text1", else: ""),
  if(data["b"] > 0, do: "text2", else: ""),
  if(data["c"] > 0, do: "text3", else: "")
])

You could even extract the if to a separate function, if the pattern if very repeating:

defp if_positive(value, text), do: if(value > 0, do: text, else: "")

IO.iodata_to_binary([
  if_positive(data["a"], "text1"),
  if_positive(data["b"], "text2"),
  if_positive(data["c"], "text3")
])

data
|> Enum.filter(&match?({_, v} when v > 0, &1))
|> Enum.map(fn 
  {"a", _} -> "text1"
  {"b", _} -> "text2"
  {"c", _} -> "text3"
end)

A more functional apporach

output_texts = %{
  "a" => "text1",
  "b" => "text2",
  "c" => "text3",
}

out = data
      |> Enum.filter(fn {_, v} -> v > 0 end)
      |> Enum.filter(fn {k, _} -> Enum.member(Map.keys(output_texts), k) end)
      |> Enum.map(fn {k, _} -> output_texts[k] end)
      |> Enum.join()