I have a e350 1996 300 cu since a few month.
I have read somewhere that the c6 is 3 speeds + reverse. I can clearly feel the trans shifting from 1 to 2 but I have never really felt 2 to 3.
Some people mention doing it manually but on my shifter l only have P R D 1 2 no 3
It is also my first automatic trans so I am quite clueless.

 

In a C6 your D is the 1,2,3 progression. If you select 1, you are in 1st gear fulltime with no shifting out. If you select 2, you begin in 2nd gear and when appropriate it shifts to 3rd. If you come to a stop, it begins again in 2nd. This can be helpful for soft starts in snow or sand.
If you are in 2, this also has a "hill-holder" feature that will not allow the vehicle to roll back even if you take your foot off the brake.
1st to 2nd is done with a band applied and can be more noticeable than 2nd to 3rd which is a clutch applied for the shift

 

There is no way to manually select 3. When you put the shift handle in 2 it will always be in 2, it will not ever shift to 3 or 1. When in 1 it always stays in 1. But there is no way to keep it in 3.

 

To see if it is shifting 2-3, get the van moving in either 1 or 2, then put it in 2 and get a feel for engine RPM thru sound. Then put it in D. If you feel and hear a shift, that will be the shift into 3rd. If D and 2 feel like the same gear, it is not shifting into 3rd.

 

Thank for all the response, I did try different things, put in 1 and 2 then fell the rpm and then switch to D to feel the difference, all speed seems to work 1 to 2 switch beetween 5 to 10 mph, then 2 to 3 switch around 25/30 mph