To manage your lawn effectively before overseeding, it's recommended to let your grass grow until it reaches a height of about 4-6 inches before cutting. Cut only one-third (1/3) of the height of the grass blades when you do mow. Since your lawn is currently at 3 inches, you should allow it to grow to around 4-5 inches before considering your next cut.

A rule of thumb is to wait until your grass reaches this height, which typically takes about 1-2 weeks under ideal conditions, depending on factors like weather, sunlight, and moisture. However, if your lawn is healthy and growing well, you might be able to mow sooner if you follow the one-third rule and maintain overall grass health during overseeding.

Please make sure to prepare your lawn correctly before overseeding as well, ensuring good soil contact and ideal conditions for seed germination.
https://yardandgarden.extension.iastate.edu/how-to/overseeding-lawn
https://extension.umn.edu/lawn-care/renovating-lawn-quality-and-sustainability See lawn reunovation section. 

It was at 4” ht and I cut it to 3” ht. Now I’m trying to get it at 1” ht to overseed it. my question was how many days should I wait to cut it again. Per UMN extension the lawn needs to be at 1” HT to allow the seeds to receive sunlight. 
Laura



Your reply was caught in a system wide computer problem. It is not the number of days, but the speed which the grass will grow depending on the weather. Hope your were able to solve your issue.