If your 3rd pillar is invested into stock market funds, as it should be, you ARE invested in the stock. There is no dichotomy.
I think in your situation the most important thing is to start. If you are not sure about 3rd pillar, start investing in a taxable brokerage account, get some experience of been invested and learn/evaluate 3rd pillar better.
Starting from the next year, you should be able to contribute to previous years in 3a. Might be actually an excellent opportunity to pass now and invest in 5 (?) years when you should have a higher income.