Divine Dragon - Excelion reminds me a lot like Cyber Blader. You only get one of the effects. However, if multiple copies do exist in the Graveyard, you can pick multiple effects; however, just not the same one. The part where it says, "These effects cannot accumulate." mean that you can't use the first effect a total of three times or the first effect and then the third effect twice. It just doesn't work like that.
If two copies of Divine Dragon - Excelion exist in your Graveyard, which is part of Divine Dragon - Excelion's effect, and you normal summon your last copy, you get two of the effects. One of each Divine Dragon - Excelion in your Graveyard. This is printed right on the card btw.
Just remember that the part that says, "These effects cannot accumulate." is referencing to you trying to use the same effect more than once for that copy.
However, you'll never get all three effects at once unless you use Exchange, steal your opponent's copy with three of yours in your Graveyard.
Btw, I'm just backing up Jedi. XD