Having problems trying to add multiple objectives to sidebar using command blocks in minecraft

There can only ever be one objective set to display on the sidebar. What you can do however, and what you seem to be trying to do, is have one score displayed, then set this score for different "fake players" to your scores in the different objectives. Note that this will only work for one player.

It'll look something like this, where Scores is the one set to display in the sidebar:

Note that I've chosen different names for the objective name (e.g: Deaths), the objective type (e.g: deathCount), and the fake player's name (e.g: Number_Of_Deaths).

You'll first need to add all of the objectives that you'll be using:

/scoreboard objectives add Deaths deathCount
/scoreboard objectives add Kills totalKillCount
/scoreboard objectives add FlightDistance stat.flyOneCm
/scoreboard objectives add Scores dummy

On a clock, you'll then want to run these commands which set the fake player's score to your score in the corresponding objective:

/scoreboard players operation Number_Of_Deaths Scores = Rehunter Deaths
/scoreboard players operation Number_Of_Kills Scores = Rehunter Kills
/scoreboard players operation Distance_Flown Scores = Rehunter FlightDistance

This should then make the sidebar look something like: