You can make a 3 x 3 collage by starting with a 3 x 1 layout (or 1 x 3 layout), reduce the border width to zero (in style menu), add first 3 videos, and share to Gallery. Repeat this for the next 3 videos, and final 3 videos (a total of 9 videos). Then finally, start a new project with a 1 x 3 layout (or 3 x 1 layout) and add the 3 saved videos from Gallery to result in your 3 x 3 layout with a total of 9 videos.
You can use this approach to multiply your videos from a 6 frame layout up to 6 x 6 (for a total of 36 videos).
If you want to use the PRO 3 x 3 frame, you can purchase a monthly or yearly subscription to upgrade to the PRO version.
Press the PRO icon at the top, and choose "PRO Editing - Monthly Subscription" or "PRO Editing - Yearly Subscription" and follow the in-app purchase steps. With the subscription you get to remove all advertisements from the app, import up to 5 minute videos from gallery, use the 3 x 3 frame, and personalize the watermark with your own name/text (up to 20 characters).