Third grade is an important time to learn study skills, because work load increases significantly in fourth grade and continues as such. First off, she has to feel no pressure and have fun while learning. Yes, reading, math, science, etc should be fun enough to capture her curiosity and motivate her to absorb the information she is learning. Personally, I feel she is doing well and as she gets older, she will most likely improve. Children study better in different environments. My 8 year old likes it quiet with me sitting beside her when questions arise and my 12 year old likes an i pod with ear phones to help him concentrate( He is an honor student in all classes and in advance reading and math). It sounds like your daughter understands and studies well, but perhaps gets nervous during her tests( you mentioned: "she loses marks in the exam". Try talking to her about this and see if you can help her prepare for the tests without pressure. You may have to review her homework with her: e.g. take practice spelling test at home, ask her questions from science and social studies before tests and give her extra math questions to practice during week end and some free time. Do this only for about 15 minutes, so as not to over whelm her and not all at the same time. I also give positive feedback, some times small "rewards" and sometimes withhold minor privileges until work is complete. Both my husband have done this with my son and are currently doing this with my daughter in a casual way, but also do fun things with them: both are involved in several sports and music but also take time for movies, parks, museums, and free play for a healthy balance. Hope this was helpful.
When she is at home, have her practice reading the questions and problems aloud -- not every single one, but enough to form the habit of reading every single word. My students whose test grades don't match their abilities get problems wrong because they skim. They think they understand the question, because they do understand things quickly. Unfortunately, they skip important words. Another factor is the "good enough" factor -- if I can get the lowest in the A range, it's good enough. I might suggest the whole family, including the parents, set improvement goals. "Our car looks good enough, but I'm going to show pride by cleaning the inside out once a week." Etc. If the whole family is setting goals and reaping a reward (perhaps a pizza night once everyone in the family has earned a pizza token for reaching a goal), your daughter may be more interested in concentrating during tests.