How do I get Turbotax to use the 3 year GDS MACRS schedule for my race horses?

You are correct. The steps below will show you how to arrive at the correct recovery period of 3 years GDC MACRS.

3-year property.

• Tractor units for over-the-road use.
• Any race horse over 2 years old when placed in service.
• Any other horse (other than a race horse) over 12 years old when placed in service.
• Qualified rent-to-own property (defined later).

Steps to entry of your race horse:

1. Sign into your TurboTax return > Search (upper right) > Type schedule c > Click the Jump to... link> Edit or Update beside your business
2. Scroll to Assets > Add or Update your Asset > On the page 'Describe this Asset' select Intangibles, Other property > Continue
3. Select Other asset type > Enter the details of your asset/race horse > Continue > Check the box 'I purchased this asset and Yes I've always used this item 100% of the time for business'
4. Enter the date you first started using it for business > Continue > Select Asset class 3 year > Continue > Select 200% Declining Balance > Continue > 
5. Select No 'Does this asset qualify as listed property' > Select 'I'll spread the deduction over several years (or other selection) > Continue
6. Arrive at the Asset Summary > Click Show Details to see your depreciation calculations