There are a number of issues that can cause failure to engage the direct (3rd) gear set. The easy to check include the governor, fluid level, and filter condition. After that the valve body (rare) or something internal is leaking enough the system can't get enough pressure to apply all the clutches. The Bentley details some tests that can be made with a pressure gauge. Start with fluid level and condition and if needed replace the fluid and clean-replace the filter. 
 
Dennis
 
 
> Date: Wed, 11 May 2016 10:32:17 -0500
> From: jrodgers113@GMAIL.COM
> Subject: Autotranny
> To: vanagon@GERRY.VANAGON.COM
> 
> What would be some causes for the autotranny not to shift into 3rd gear. It
> worked fine before but has been sitting for 12 months!
> 
> John