Yes, that's what I've been telling you.
It is not.
PS1 memory cards can only be used on PS1 games, even when playing them on a PS2 (or a PS3, which you can also do); these games stored files as "blocks", with each card split up into 12 blocks. No matter how little data you saved, it would save as a block.
PS2 memory cards can only be used on PS2 games (even when playing them on a PS3, which you can also do); these games stored files as data and you could save as many files as you wanted from as many games as you wanted so long as you didn't exceed the 8MB card size limit (some aftermarket cards were larger).
GT3 on PS2 cannot and will not read GT1 or GT2 data from a PS1 memory card.
Apparently you like to not read links before posting them.
Just as the GTPlanet link you posted directly refuted your claim that you could transfer garage data from one game to the next, this Wiki link you've posted makes no mention of transferring garage data from one game to the next. Trade functions only operate within the same game; you could trade GT2 garage cars to another GT2 garage, GT5 garage cars to another GT5 garage, and GT PSP garage cars to another GT PSP garage.
You have never been able to trade GT1 garage cars to a GT2 garage, or GT2 garage cars to a GT3 garage, or any GT game garage cars to any different GT game.
For some reason you're remembering something wrongly and getting foot-stampingly, capital letter cross about how right you are... without checking that you are. Had you checked you'd discover that you aren't.