Here is my tips on how to solo or with only 3 people, this is how i always have survived.
1. Join the biggest company you can find
2. Use them to level up to 70-100+
3. Ditch them and venture alone or with a few friends.
4. Find a island that has alot of mountains and good enough resourcers.
5. Find a very hidden spot ontop of a mountain, if there is also trees up there which also can hide. Build your secret main base there
6. Build some storages under water. People never look for these unless they know you have one.
7. If you are not alone build nothing more than a schooner, and if you are alone only build sloops! People usally don't waste effort destroying sloops so they are good to have many of. Make sure that they have either small speed, small weight or medium speed.
8. Go out in the world and raid the shit out of everyone you see. Take all their loot and hide it somewhere. Under water or behind a bush. Then later you come with your sloop and pick them up.
9. Make a great company name and similar stuff that will make people think your company is bigger than it is.
10. Form alliance with nearby solo companies, it can become useful having some allies so dont kill absolutely everyone you see:c
11. Make a decoy main base. In case you get attacked you build a fake base. Make it out of stone and do some effort to build it. Then place alot of chests, tables, chairs, furniture, make it look like this is your home. Just have a few of your resourcers there, not much. You will really just use this place to storage resourcers you find before moving it to your real base. Now when they have destroyed your fake base they think that they have completely ruin you. Now this gives you breathing area and can assess the situation. When its safe rebuild your decoy.
12. Become a settler of another island is also an option. Then you can be safe from anyone but the enemies of the company you are under. You don't have to worry about getting hunted down if you attack some other companies. Remember to have a very good relations with the company you are under though.
I think this covers most of it.