Can you tell me the three critical KPIs that I can bring to the CMO or CFO to prove the value of PMM?

There are two questions buried in here. 

If you are truly in a situation where your leadership doesn't understand the impact of Product Marketing, are you in the right position? Product Marketing is challenging enough when you have buy-in and support from your leaders. I'd question why this is happening - perhaps you need to do some education around what is and what is not PMM.

I can answer this one best from an enterprise/B2B point of view. At the end of the day, the role of PMM in a B2B sales motion is threefold: deliver "higher quality" or more informed leads into the pipeline, accelerate the sales cycle, and increase average deal size. That maps to three critical (and shared) metrics to really show both immediate impact and long-term strategic value:

1. Meetings Set. Whether you call these SAOs, opportunities created, or just sales-accepted meetings, this is a critical leading indicator of PMM fit. Basically, are you getting more educated leads to the point of sale, and is sales accepting them? More at bats = more opportunities to sell, and more chances to learn.
2. Pipeline Generated. Own this number above all else. How much net new or incremental opportunity are PMM programs driving into sales. Allows you to measure contribution, but also effectiveness. How much coverage do you need to hit the number? How well do those opps convert?
3. Average Deal Size. This is related to Pipe Generation, and plays out over a longer period, but this is the sales metric that PMM can have the biggest impact on over time. Pricing, packaging, new product introduction, sales enablement, and positioning. Ask yourself: what milestones can we hit if we increase deal sizes by 15%? By 50%?

Without knowing your business or business model, I think your CMO has a few key areas they are focused on, and they are looking to understand how you are going to help influence these areas.
1. Leads. All CMO's are tasked with delivering qualified leads to the sales organization and they need everyone in the marketing organization working towards delivering qualified leads. Figure out how your PMM team is going to contribute to hitting this number.
2. Brand awareness. This can be via your marketing website, digital ads, physical ads, social presence, etc - your CMO wants your organizations brand to be recognized and for that awareness to convert into... Leads (see #1). So make sure your PMM team has KPIs tied to the messaging and positioning that feeds into your brand awareness and strategy.

3. Sales support. Your CMO is being held accountable by your head of Sales to ensure sales has everything they need to convert those marketing leads. Therefore, make sure your team has an enablement strategy that brings the positioning and messaging of your product to the seller to create a throughline from the top of the marketing funnel to the sales funnel.

Your CFO wants to keep costs low, and revenue high. Any software your team uses to help do your job (competitive intelligence tools, content management systems, etc) - be sure to demonstrate ROI. And as part of #3 above, tie yourself to revenue anyway you can. How has what you launched, or created for sales, etc led to business growth.

What KPIs you bring to the CMO or CFO to prove the value of PMM depends upon which metrics you can directly attribute to PMM in your organization. Some organizations have attribution models that let them directly attribute pipeline and revenue to marketing-driven activities. Here are some quantitative and qualitative metrics you can use:

Sales team enablement/engagement: PMMs are directly responsible for enabling the field on products. Any metrics around # of AEs/SEs/SDRs trained, usage metrics on enablement content, pipeline created X days after an enablement, or qualitative feedback from sales teams will prove PMM's value.

Content performance: PMMs create demos, webinars, datasheets, guides--any content created by PMMs can be tracked. Does performance often rely on how well it's also promoted? Yes. But especially if you can track webinar attendance and completion rates, downloads on content, or even how often sales teams are using this content with customers, that will be a direct reflection of PMM's value to the organization.

Pipeline and revenue impact: If your organization has an attribution model enabling you to tie marketing activities to pipeline or revenue, that's the golden ticket. If not, you could look at pipeline and revenue generated by specific individuals/teams/accounts who engaged with a marketing activity/activation For example, opportunities opened or closed after an event; accounts that attended a webinar or downloaded a piece of content.

Website and Campaign Performance: While PMMs might not directly own web or campaigns, we do own the message and influence the strategy. How does A/B testing on campaign messages impact performance? What about web metrics after improvements and updates? It's all fair game!