Preparing to enter ihraam approximately three hours before reaching the meeqaat

Yes, that is permissible, but you should not enter ihraam until you pass the meeqaat. 

Shaykh Ibn Baaz was asked about someone who lives 350 km from Makkah and comes to Makkah by car – is it permissible for him to prepare for ihraam and do ghusl and put on the ihraam garments in his city? Is it permissible for him to enter ihraam from his city?

 He replied: 

There is nothing wrong with doing ghusl and putting on the ihraam garments and perfume in their homes because they are close to the meeqaat by car. But it is prescribed for them to enter ihraam only from the meeqaat. Ihraam means the intention to start the rituals (of Hajj or ‘Umrah). Along with forming the intention it is prescribed to utter these words such as saying “Labbayk ‘umratan (Here I am for ‘Umrah)” or “Labbayk Hajjan (Here I am for Hajj,” then he should recite the Talbiyah as prescribed in sharee’ah, which is: “Labbayka Allaahumma labbayk, labbayka laa shareeka laka labbayk. Inna al-hamd wa’l-ni’mata laka wa’l-mulk, laa shareeka lak  (Here I am, O Allaah, here I am. Here I am, You have no partner, here I am. Verily all praise and blessings are Yours, and all sovereignty, You have no partner).”

 May Allaah help us all to do that which pleases Him.

 Fataawa Ibn Baaz, 17/51.