Embed a 3D viewer in your Instructable
It's pretty simple:
For Tinkercad - on your model's pop up page, there is a "Share" section with an Embed, Facebook, Twitter and Pinterest buttons. Click the embed button and copy the code.
For 123D, the Embed button is on the upper right-hand side of your model's edit page in the 123D Gallery. In the pop-up, select the second, longer code to embed.
Once you have the code, go to your Instructable draft. You could post the viewer as if it were a video - in the buttons above the text box, there is a little filmstrip icon. Click it, paste the code and click OK.
OR: If you have a Pro Acct, you'll see a "Source" button above the text box. Click it and paste the code in line wherever you want to to go in your text.
Either way, BAM. Now you have a 3D image that fully shows off your completed project..!