I'm worried about my granddaughter who is 3.5 months. She is not even wanting to use her legs to stand up when we try to make her stand. But I can see when she's laying down she moves her legs, and when she's mad she kicks very strong. Do we need to ask her doctor about this?
Three and a half months is much too early to be trying to stand up. Most babies don't start to stand up whilst holding something until around 7-9 months although they may be putting weight on their legs before then with help. 3.5 months is even very early for sitting up.
Here is a link for what you might expect babies to be doing at any particular age. "Can bear weight on legs" comes at 4 months but "stands while holding onto something" is listed as an advanced skill at 7 months
Even so, all children develop at their own pace with their own strengths and weaknesses. Three and a half months seems way too soon to be putting pressure on a baby to be making any big achievements. Just enjoy your granddaughter while she is tiny - it doesn't last long!