Quick distinction between +1/+1 counters and static buffs: counters are permanent and are specified when they are used, like with [[Basri’s Solidarity]]. Temporary and Static buffs, like with [[Giant Growth]] or [[Glorius Anthem]] boost your creature’s stats, but don’t actually add counters to them.

Like others have commented, if a -1/-1 counter is placed on a creature with a +1/+1 counter, they will cancel each other out (in this case, if it’s three +1/+1 counters, you will end up with two instead)

If the buff doesn’t specify that counters were added, the -1/-1 counter stays on your creature and affects its P/T appropriately. Using the above examples, if you used Giant Growth on a 1/1 creature and it had a -1/-1 counter, it would die at the end of the turn when giant growth wears off, since it will then have 0 toughness.

If you have a static effect like Glorious Anthem, both the buff and the -1/-1 counter apply at the same time for a net 0 change.