My 3-month-old has only recently started using a dummy due to being unsettled this week, possibly from a leap. We hadn't used one before, but now offer it if she doesn’t fall asleep after 5–10 minutes of cuddling. I’ve read that if using a dummy, it should be used for all sleeps. Is it unsafe to only offer it sometimes , or is it ok to continue seeing if she falls asleep herself after 5-10 mins of cuddles/after a bottle etc?

There are 2 main reasons that parents use a dummy for baby babies.
The most common reason is to assist settling, as you have chosen to use.

Secondly, Red Nose information about dummy use is mainly related to use of a dummy as a risk reduction for Sudden Unexpected Death in Infancy (SUDI)
There is some evidence to support the use of a dummy as a risk reduction for SUDI, however, Red Nose does not promote the use of a dummy as part of Safe Sleep recommendations. (Some countries eg USA have added to recommendations, other countries haven’t)

If you choose to use a dummy for risk reduction, then it needs to be used for all sleeps.
If parents are using a dummy as a settling techniques, then it can be used as required.

https://rednose.org.au/article/does-dummy-use-reduce-the-risk-of-sudden-unexpected-death-in-infancy