Pls guys I need help with mine, pls I need ur advice my ogas

I want to use this priviledge to salute the able elders in this forum for this great thread, God bless you and give you all your heart desires. Please i need your help on this issue, I am a 28 years old Nigeria planing to migrate to Australia, Am a Geologist and when checking through the criteria for Visa type sub class 190 I was bothered on how to go about with the work experience, I finished in 2015 and I started practicing as a geologist in a geological firm on contract base, i got paid per work mostly with direct hand to hand cash so no black and white proof of payment, 1n 2016 i left the company to registered my own company and i started practicing till date. my question now is
1. Can I add all these experience together because that is when i can have the minimum 3 yrs experience required
2. What proof can i collect from my previous employer because he is ready to help in any form
3. What proof can i Present for practicing as geologist on my own. I have my certificate of registration with the corporate affairs commission payment of annual returns receipt, am registered with Federal inland revenue, have a Tin number for my company and few VAT receipt for this year only. Thanks