Plan for generating external funding
What I would expect here is a (short) document where you describe how you plan to build up your lab in the next three years. Which personal grants to you plan to target? Do you have multiple options, or will you be all-in on one specific grant? Are there specific project calls or larger consortia where you could apply, and what makes you a good person to apply for them (i.e., this is a good place to brag about your network for participating in larger collaborations)? Are there other types of grants (for infrastructure, etc.) that are required or useful for the kind of work that you do? How will the different types of grants connect to form a larger research agenda?

All in all, I expect that the committee really wants to see (a) that  there are funding sources available that you can realistically apply to - ideally, these will be a combination of high-risk/high-reward calls (e.g., highly prestigious personal grants) and more conservative projects, and (b) that you are aware of the different funding sources and have a plan in mind how you will use them to build up  your group. Everybody understands that this is, at best, a declaration of intent - grants get rejected, new opportunities arise, and your research interests shift, so nobody will fault you if you, 3 years from now, have deviated greatly from this plan.