[hand drive]

it looks like those two different beam configurations are very close to the same strength while in comparison to one another.

 

[hand drive]

http://www.awc.org/Publications/DCA/DCA6/DCA6-09.pdf

 

[hand drive]

So then the 2x8 is stronger than the triple 2x6 beam.

The pdf says the triple 2x6 is stronger

for spf the 3-2x6 beam is stronger than 2-2x8 and for southern yellow pine the 2-2x8 is stronger than the 3- 2x6.

in order for the triple 2x6 to be stronger it has to be nailed in the correct beam nailing pattern and if your floor joists for the deck are 2x8 then a triple 2x6 beam does not work to well.

 

[hand drive]

I am going to use 2x8 for the joists. Why would the triple 2x6 not work as well?

That's what I was planning to user since it was stronger with less height. It was perfect...

oops, forgot to say. if the beam is flush with the joists ( flush beam ) then it would not work but if it is a dropped beam then no problem.