Thank you for contacting Ask Extension. None of the three plants that you mentioned like to be in standing water. If the area drains within a 24 hour period, then all is good and they will handle that area. But if it takes longer than that, then you might want to consider moving them. When plants are in standing water for prolonged periods, they can experience stunted growth, especially if they are young or seedlings. In your case, the hydrangeas and lilacs, being small bushes, may be more susceptible to damage due to the flooding. 

It's also important to avoid working with plants when they are wet to prevent the spread of diseases. In terms of what you can do, you should avoid pruning or fertilizing the plants as this could cause further stress.  If the water has not receded after a few days, you may need to consider replanting the hydrangeas and lilacs, as they might not survive the prolonged flooding. 

Remember, once damage is done, it can’t be fully reversed, but plants will grow again if they were not too severely damaged. It’s best to remove all fruits and flowers from the plants to encourage vegetative growth, which can help offset the negative effects of the flooding.

Here are some references for you:

https://www.aces.edu/blog/topics/emergency-handbook-home-family/flooded-gardens-and-fields/

https://extension.umd.edu/resource/excess-water-damage-flowers/