Lots of good info in this thread already, and I don't have much I can offer to expand on the what's and why's.
As I was a neophyte in a similar situation a couple of months back I thought I could add a few points from the perspective of doing this for the first time out.
I had the opportunity recently to take some live performance music shots as a volunteer at a music festival on two nights at two different venues. My most current body is a K-r, and my fastest lens options are a couple of 50 1.7's, the M and an F. I have done lots of music performance photography at outdoor festivals, but this is the first time I have ever shot live performance indoors. The images are for my own enjoyment, although I have shared some of them with the performers.
I had read through other posts on the site to see what kind of tips were suggested, and decided to go with my F 50 1.7 on the K-r, and as a backup I took my K200D with A 135 2.8 as I am comfortable with that combo.
The post immediately above saying push the ISO as far as you can is a statement I would certainly endorse and something I will do next time I'm in a similar situation. I started out the first evening on straight Auto, then as I got a feel for the light and the kind of results I was getting I started trying different ISO and settings combinations. From looking at my straight out of the camera keepers, I suspect you will want to crank up the ISO.
I am glad I took the K200D along with me as a secondary. The first night I very briefly tried a few shots on the K-r with my 18-135 but put that back in the bag right away. So, started out the first night getting a few shots on the K200D with the F50 before I had a break in my duties to swap. The second night I got so trigger happy I needed to pull the K200 out as I had brought both of my K-r batteries but had forgotten my AA battery carrier at home. Using catch in focus with a manual lens is handy. I expect that as you're a manual lens user that you're comfortable with your focus options on the K-3 but if you're not familiar how it works, maybe do some test shooting before heading out.
I found that the 50 was a good focal length for the first venue where the performers were a solo act, trio and four piece on a tight stage. At the main stage for the second night, a wider lens would have been handy with six musicians and three of them out front singing. You have the advantage of already knowing the band, but you may want to pack along the 35 or 40 just in case.
I've included links to a few JPEG images I had already tossed in other threads just for comparison purposes to what has already been posted in this thread. K200D F50, K-r F50, K200D A 135. No cropping, no PP so you can have a look at the EXIF values if you like.
https://www.pentaxforums.com/forums/attachments/122-lens-clubs/430937d154247...b-imgp7995.jpg https://www.pentaxforums.com/forums/attachments/122-lens-clubs/431157d154263...b-imgp6414.jpg https://www.pentaxforums.com/forums/attachments/6-pentax-dslr-discussion/431...d-imgp6434.jpg