Thanks, Metro, this is my first win. I had a lot of fun with this one. Here's how I did it. Please forgive my long-windedness...
Completed Collections:
Fish (3)
Frogs (3)
Gardening (3)
Geodes (3)
Space prints (3)
Fossils (1)
All three of the sims were:
Geek
Insider
Loves outdoors
Upon initial setup, I put sims 2 and 3 on the two 15x20 lots in Newcrest. Sim 1 moved to the 30x40 lot in the same neighborhood (I planned to grow a large garden on his lot and needed the room). The two 15x20 lots (private dwelling only) were used as storage for things I couldn't sell (buy mode items caught while fishing, things crafted with the workbench, etc). The 30x40 lot (private dwelling, science lair, great soil) had a shower room, a spaceship, a telescope, workbenches, and a large garden.
I setup a club for my three sims with club activities of friendly socials and fishing. Rally troops was my first club perk followed by skill boosts for handiness and rocket science and focusing boosts.
In the first few rotations, the club would go to various places to fish, collect frogs, harvest plants, and dig for metals, fossils, crystals and MySims. Then go back to the 30x40 lot to plant at least one of each type of plant. I also got produce from starter seeds, attending the first Spice and Romance festivals, one space mission, grafting, and letting a pile of trash take root.
I wanted to get to Sixam for two things, the geode collection (usually takes only two trips) and the alien plants (quill and glow orb). Therefore, I had sim 2 max rocket science skill in order to upgrade the spaceship to travel there.
I know that the quill and glow orb plants are not required to complete the gardening collection, however, I used them as cash crops. Those two plants start at §20 when they are normal quality; at perfect quality, they are worth just over §100 a piece. Another deciding factor was how fast they grow (time between full harvests for those is about 2 sim days). I also grew dragon fruit (§16-80/each) as a cash crop, but their harvest cycle was a bit longer (about 3 sim days). In total, the garden had over 90 plants. To care for it, I used the "tend garden together" option regularly. I never had gardening as a club activity because the produce would be harvested early (less profitable) and those harvested by the two sims I wasn't controlling would very likely be lost (not profitable at all).
As for completing the fish collection I spent lots of time out at various locations to catch everything. Sim 3 maxed handiness to break open the path to the Forgotten Grotto. The most variety was caught in and around the Oasis Springs Park.
I got the vast majority of frogs from the 3 ponds (Willow Creek Park, Oasis Springs Park, and Sylvan Glade). The rest I bred strategically to collect. Once I had finished the collection, I continued to breed the most expensive frogs. This would add approx §2,000 per sim per rotation to household income.
As I made my way around the different neighborhoods, I dug all the rocks. Sim 1 managed to complete the fossil collection in time. I got close on some of the others but it wasn't in the cards.
The last several rotations were spent entirely on sim 1's lot so the garden could grow. To pass the time in between gardening duties, I collected space prints with the telescope. I found that you got more variety if every time you collected one, you cancel the interaction, go do something else, and then come back.
Thanks to the team for another fun challenge!